Provided by: ocfs2-tools_1.8.7-1build4_amd64 
NAME
OCFS2 - A Shared-Disk Cluster File System for Linux
INTRODUCTION
OCFS2 is a file system. It allows users to store and retrieve data. The data is stored in files that are
organized in a hierarchical directory tree. It is a POSIX compliant file system that supports the
standard interfaces and the behavioral semantics as spelled out by that specification.
It is also a shared disk cluster file system, one that allows multiple nodes to access the same disk at
the same time. This is where the fun begins as allowing a file system to be accessible on multiple nodes
opens a can of worms. What if the nodes are of different architectures? What if a node dies while writing
to the file system? What data consistency can one expect if processes on two nodes are reading and
writing concurrently? What if one node removes a file while it is still being used on another node?
Unlike most shared file systems where the answer is fuzzy, the answer in OCFS2 is very well defined. It
behaves on all nodes exactly like a local file system. If a file is removed, the directory entry is
removed but the inode is kept as long as it is in use across the cluster. When the last user closes the
descriptor, the inode is marked for deletion.
The data consistency model follows the same principle. It works as if the two processes that are running
on two different nodes are running on the same node. A read on a node gets the last write irrespective of
the IO mode used. The modes can be buffered, direct, asynchronous, splice or memory mapped IOs. It is
fully cache coherent.
Take for example the REFLINK feature that allows a user to create multiple write-able snapshots of a
file. This feature, like all others, is fully cluster-aware. A file being written to on multiple nodes
can be safely reflinked on another. The snapshot created is a point-in-time image of the file that
includes both the file data and all its attributes (including extended attributes).
It is a journaling file system. When a node dies, a surviving node transparently replays the journal of
the dead node. This ensures that the file system metadata is always consistent. It also defaults to
ordered data journaling to ensure the file data is flushed to disk before the journal commit, to remove
the small possibility of stale data appearing in files after a crash.
It is architecture and endian neutral. It allows concurrent mounts on nodes with different processors
like x86, x86_64, IA64 and PPC64. It handles little and big endian, 32-bit and 64-bit architectures.
It is feature rich. It supports indexed directories, metadata checksums, extended attributes, POSIX ACLs,
quotas, REFLINKs, sparse files, unwritten extents and inline-data.
It is fully integrated with the mainline Linux kernel. The file system was merged into Linux kernel
2.6.16 in early 2006.
It is quickly installed. It is available with almost all Linux distributions. The file system is on-disk
compatible across all of them.
It is modular. The file system can be configured to operate with other cluster stacks like Pacemaker and
CMAN along with its own stack, O2CB.
It is easily configured. The O2CB cluster stack configuration involves editing two files, one for cluster
layout and the other for cluster timeouts.
It is very efficient. The file system consumes very little resources. It is used to store virtual machine
images in limited memory environments like Xen and KVM.
In summary, OCFS2 is an efficient, easily configured, modular, quickly installed, fully integrated and
compatible, feature-rich, architecture and endian neutral, cache coherent, ordered data journaling,
POSIX-compliant, shared disk cluster file system.
OVERVIEW
OCFS2 is a general-purpose shared-disk cluster file system for Linux capable of providing both high
performance and high availability.
As it provides local file system semantics, it can be used with almost all applications. Cluster-aware
applications can make use of cache-coherent parallel I/Os from multiple nodes to scale out applications
easily. Other applications can make use of the clustering facilities to fail-over running application in
the event of a node failure.
The notable features of the file system are:
Tunable Block size
The file system supports block sizes of 512, 1K, 2K and 4K bytes. 4KB is almost always
recommended. This feature is available in all releases of the file system.
Tunable Cluster size
A cluster size is also referred to as an allocation unit. The file system supports cluster sizes
of 4K, 8K, 16K, 32K, 64K, 128K, 256K, 512K and 1M bytes. For most use cases, 4KB is recommended.
However, a larger value is recommended for volumes hosting mostly very large files like database
files, virtual machine images, etc. A large cluster size allows the file system to store large
files more efficiently. This feature is available in all releases of the file system.
Endian and Architecture neutral
The file system can be mounted concurrently on nodes having different architectures. Like 32-bit,
64-bit, little-endian (x86, x86_64, ia64) and big-endian (ppc64, s390x). This feature is
available in all releases of the file system.
Buffered, Direct, Asynchronous, Splice and Memory Mapped I/O modes
The file system supports all modes of I/O for maximum flexibility and performance. It also
supports cluster-wide shared writeable mmap(2). The support for bufferred, direct and asynchronous
I/O is available in all releases. The support for splice I/O was added in Linux kernel 2.6.20 and
for shared writeable map(2) in 2.6.23.
Multiple Cluster Stacks
The file system includes a flexible framework to allow it to function with userspace cluster
stacks like Pacemaker (pcmk) and CMAN (cman), its own in-kernel cluster stack o2cb and no cluster
stack.
The support for o2cb cluster stack is available in all releases.
The support for no cluster stack, or local mount, was added in Linux kernel 2.6.20.
The support for userspace cluster stack was added in Linux kernel 2.6.26.
Journaling
The file system supports both ordered (default) and writeback data journaling modes to provide
file system consistency in the event of power failure or system crash. It uses JBD2 in Linux
kernel 2.6.28 and later. It used JBD in earlier kernels.
Extent-based Allocations
The file system allocates and tracks space in ranges of clusters. This is unlike block based file
systems that have to track each and every block. This feature allows the file system to be very
efficient when dealing with both large volumes and large files. This feature is available in all
releases of the file system.
Sparse files
Sparse files are files with holes. With this feature, the file system delays allocating space
until a write is issued to a cluster. This feature was added in Linux kernel 2.6.22 and requires
enabling on-disk feature sparse.
Unwritten Extents
An unwritten extent is also referred to as user pre-allocation. It allows an application to
request a range of clusters to be allocated, but not initialized, within a file. Pre-allocation
allows the file system to optimize the data layout with fewer, larger extents. It also provides a
performance boost, delaying initialization until the user writes to the clusters. This feature was
added in Linux kernel 2.6.23 and requires enabling on-disk feature unwritten.
Hole Punching
Hole punching allows an application to remove arbitrary allocated regions within a file. Creating
holes, essentially. This is more efficient than zeroing the same extents. This feature is
especially useful in virtualized environments as it allows a block discard in a guest file system
to be converted to a hole punch in the host file system thus allowing users to reduce disk space
usage. This feature was added in Linux kernel 2.6.23 and requires enabling on-disk features sparse
and unwritten.
Inline-data
Inline data is also referred to as data-in-inode as it allows storing small files and directories
in the inode block. This not only saves space but also has a positive impact on cold-cache
directory and file operations. The data is transparently moved out to an extent when it no longer
fits inside the inode block. This feature was added in Linux kernel 2.6.24 and requires enabling
on-disk feature inline-data.
REFLINK
REFLINK is also referred to as fast copy. It allows users to atomically (and instantly) copy
regular files. In other words, create multiple writeable snapshots of regular files. It is called
REFLINK because it looks and feels more like a (hard) link(2) than a traditional snapshot. Like a
link, it is a regular user operation, subject to the security attributes of the inode being
reflinked and not to the super user privileges typically required to create a snapshot. Like a
link, it operates within a file system. But unlike a link, it links the inodes at the data extent
level allowing each reflinked inode to grow independently as and when written to. Up to four
billion inodes can share a data extent. This feature was added in Linux kernel 2.6.32 and
requires enabling on-disk feature refcount.
Allocation Reservation
File contiguity plays an important role in file system performance. When a file is fragmented on
disk, reading and writing to the file involves many seeks, leading to lower throughput. Contiguous
files, on the other hand, minimize seeks, allowing the disks to perform IO at the maximum rate.
With allocation reservation, the file system reserves a window in the bitmap for all extending
files allowing each to grow as contiguously as possible. As this extra space is not actually
allocated, it is available for use by other files if the need arises. This feature was added in
Linux kernel 2.6.35 and can be tuned using the mount option resv_level.
Indexed Directories
An indexed directory allows users to perform quick lookups of a file in very large directories. It
also results in faster creates and unlinks and thus provides better overall performance. This
feature was added in Linux kernel 2.6.30 and requires enabling on-disk feature indexed-dirs.
File Attributes
This refers to EXT2-style file attributes, such as immutable, modified using chattr(1) and queried
using lsattr(1). This feature was added in Linux kernel 2.6.19.
Extended Attributes
An extended attribute refers to a name:value pair than can be associated with file system objects
like regular files, directories, symbolic links, etc. OCFS2 allows associating an unlimited number
of attributes per object. The attribute names can be up to 255 bytes in length, terminated by the
first NUL character. While it is not required, printable names (ASCII) are recommended. The
attribute values can be up to 64 KB of arbitrary binary data. These attributes can be modified and
listed using standard Linux utilities setfattr(1) and getfattr(1). This feature was added in Linux
kernel 2.6.29 and requires enabling on-disk feature xattr.
Metadata Checksums
This feature allows the file system to detect silent corruptions in all metadata blocks like
inodes and directories. This feature was added in Linux kernel 2.6.29 and requires enabling on-
disk feature metaecc.
POSIX ACLs and Security Attributes
POSIX ACLs allows assigning fine-grained discretionary access rights for files and directories.
This security scheme is a lot more flexible than the traditional file access permissions that
imposes a strict user-group-other model.
Security attributes allow the file system to support other security regimes like SELinux, SMACK,
AppArmor, etc.
Both these security extensions were added in Linux kernel 2.6.29 and requires enabling on-disk
feature xattr.
User and Group Quotas
This feature allows setting up usage quotas on user and group basis by using the standard
utilities like quota(1), setquota(8), quotacheck(8), and quotaon(8). This feature was added in
Linux kernel 2.6.29 and requires enabling on-disk features usrquota and grpquota.
Unix File Locking
The Unix operating system has historically provided two system calls to lock files. flock(2) or
BSD locking and fcntl(2) or POSIX locking. OCFS2 extends both file locks to the cluster. File
locks taken on one node interact with those taken on other nodes.
The support for clustered flock(2) was added in Linux kernel 2.6.26. All flock(2) options are
supported, including the kernels ability to cancel a lock request when an appropriate kill signal
is received by the user. This feature is supported with all cluster-stacks including o2cb.
The support for clustered fcntl(2) was added in Linux kernel 2.6.28. But because it requires
group communication to make the locks coherent, it is only supported with userspace cluster
stacks, pcmk and cman and not with the default cluster stack o2cb.
Comprehensive Tools Support
The file system has a comprehensive EXT3-style toolset that tries to use similar parameters for
ease-of-use. It includes mkfs.ocfs2(8) (format), tunefs.ocfs2(8) (tune), fsck.ocfs2(8) (check),
debugfs.ocfs2(8) (debug), etc.
Online Resize
The file system can be dynamically grown using tunefs.ocfs2(8). This feature was added in Linux
kernel 2.6.25.
RECENT CHANGES
The O2CB cluster stack has a global heartbeat mode. It allows users to specify heartbeat regions that are
consistent across all nodes. The cluster stack also allows online addition and removal of both nodes and
heartbeat regions.
o2cb(8) is the new cluster configuration utility. It is an easy to use utility that allows users to
create the cluster configuration on a node that is not part of the cluster. It replaces the older utility
o2cb_ctl(8) which has being deprecated.
ocfs2console(8) has been obsoleted.
o2info(8) is a new utility that can be used to provide file system information. It allows non-privileged
users to see the enabled file system features, block and cluster sizes, extended file stat, free space
fragmentation, etc.
o2hbmonitor(8) is a o2hb heartbeat monitor. It is an extremely light weight utility that logs messages to
the system logger once the heartbeat delay exceeds the warn threshold. This utility is useful in
identifying volumes encountering I/O delays.
debugfs.ocfs2(8) has some new commands. net_stats shows the o2net message times between various nodes.
This is useful in identifying nodes are that slowing down the cluster operations. stat_sysdir allows the
user to dump the entire system directory that can be used to debug issues. grpextents dumps the complete
free space fragmentation in the cluster group allocator.
mkfs.ocfs2(8) now enables xattr, indexed-dirs, discontig-bg, refcount, extended-slotmap and clusterinfo
feature flags by default, in addition to the older defaults, sparse, unwritten and inline-data.
mount.ocfs2(8) allows users to specify the level of cache coherency between nodes. By default the file
system operates in full coherency mode that also serializes the direct I/Os. While this mode is
technically correct, it limits the I/O thruput in a clustered database. This mount option allows the user
to limit the cache coherency to only the buffered I/Os to allow multiple nodes to do concurrent direct
writes to the same file. This feature works with Linux kernel 2.6.37 and later.
COMPATIBILITY
The OCFS2 development teams goes to great lengths to maintain compatibility. It attempts to maintain both
on-disk and network protocol compatibility across all releases of the file system. It does so even while
adding new features that entail on-disk format and network protocol changes. To do this successfully, it
follows a few rules:
1. The on-disk format changes are managed by a set of feature flags that can be turned on and off.
The file system in kernel detects these features during mount and continues only if it understands
all the features. Users encountering this have the option of either disabling that feature or
upgrading the file system to a newer release.
2. The latest release of ocfs2-tools is compatible with all versions of the file system. All
utilities detect the features enabled on disk and continue only if it understands all the features.
Users encountering this have to upgrade the tools to a newer release.
3. The network protocol version is negotiated by the nodes to ensure all nodes understand the active
protocol version.
FEATURE FLAGS
The feature flags are split into three categories, namely, Compat, Incompat and RO Compat.
Compat, or compatible, is a feature that the file system does not need to fully understand to
safely read/write to the volume. An example of this is the backup-super feature that added the
capability to backup the super block in multiple locations in the file system. As the backup super
blocks are typically not read nor written to by the file system, an older file system can safely
mount a volume with this feature enabled.
Incompat, or incompatible, is a feature that the file system needs to fully understand to
read/write to the volume. Most features fall under this category.
RO Compat, or read-only compatible, is a feature that the file system needs to fully understand to
write to the volume. Older software can safely read a volume with this feature enabled. An example
of this would be user and group quotas. As quotas are manipulated only when the file system is
written to, older software can safely mount such volumes in read-only mode.
The list of feature flags, the version of the kernel it was added in, the earliest version of the
tools that understands it, etc., is as follows:
┌──────────────────────┬────────────────┬─────────────────┬───────────┬───────────┐
│ Feature Flags │ Kernel Version │ Tools Version │ Category │ Hex Value │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ backup-super │ All │ ocfs2-tools 1.2 │ Compat │ 1 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ strict-journal-super │ All │ All │ Compat │ 2 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ local │ Linux 2.6.20 │ ocfs2-tools 1.2 │ Incompat │ 8 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ sparse │ Linux 2.6.22 │ ocfs2-tools 1.4 │ Incompat │ 10 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ inline-data │ Linux 2.6.24 │ ocfs2-tools 1.4 │ Incompat │ 40 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ extended-slotmap │ Linux 2.6.27 │ ocfs2-tools 1.6 │ Incompat │ 100 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ xattr │ Linux 2.6.29 │ ocfs2-tools 1.6 │ Incompat │ 200 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ indexed-dirs │ Linux 2.6.30 │ ocfs2-tools 1.6 │ Incompat │ 400 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ metaecc │ Linux 2.6.29 │ ocfs2-tools 1.6 │ Incompat │ 800 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ refcount │ Linux 2.6.32 │ ocfs2-tools 1.6 │ Incompat │ 1000 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ discontig-bg │ Linux 2.6.35 │ ocfs2-tools 1.6 │ Incompat │ 2000 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ clusterinfo │ Linux 2.6.37 │ ocfs2-tools 1.8 │ Incompat │ 4000 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ unwritten │ Linux 2.6.23 │ ocfs2-tools 1.4 │ RO Compat │ 1 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ grpquota │ Linux 2.6.29 │ ocfs2-tools 1.6 │ RO Compat │ 2 │
├──────────────────────┼────────────────┼─────────────────┼───────────┼───────────┤
│ usrquota │ Linux 2.6.29 │ ocfs2-tools 1.6 │ RO Compat │ 4 │
└──────────────────────┴────────────────┴─────────────────┴───────────┴───────────┘
To query the features enabled on a volume, do:
$ o2info --fs-features /dev/sdf1
backup-super strict-journal-super sparse extended-slotmap inline-data xattr
indexed-dirs refcount discontig-bg clusterinfo unwritten
ENABLING AND DISABLING FEATURES
The format utility, mkfs.ocfs2(8), allows a user to enable and disable specific features using the
fs-features option. The features are provided as a comma separated list. The enabled features are
listed as is. The disabled features are prefixed with no. The example below shows the file system
being formatted with sparse disabled and inline-data enabled.
# mkfs.ocfs2 --fs-features=nosparse,inline-data /dev/sda1
After formatting, the users can toggle features using the tune utility, tunefs.ocfs2(8). This is
an offline operation. The volume needs to be umounted across the cluster. The example below shows
the sparse feature being enabled and inline-data disabled.
# tunefs.ocfs2 --fs-features=sparse,noinline-data /dev/sda1
Care should be taken before enabling and disabling features. Users planning to use a volume with
an older version of the file system will be better of not enabling newer features as turning
disabling may not succeed.
An example would be disabling the sparse feature; this requires filling every hole. The operation
can only succeed if the file system has enough free space.
DETECTING FEATURE INCOMPATIBILITY
Say one tries to mount a volume with an incompatible feature. What happens then? How does one
detect the problem? How does one know the name of that incompatible feature?
To begin with, one should look for error messages in dmesg(8). Mount failures that are due to an
incompatible feature will always result in an error message like the following:
ERROR: couldn't mount because of unsupported optional features (200).
Here the file system is unable to mount the volume due to an unsupported optional feature. That
means that that feature is an Incompat feature. By referring to the table above, one can then
deduce that the user failed to mount a volume with the xattr feature enabled. (The value in the
error message is in hexadecimal.)
Another example of an error message due to incompatibility is as follows:
ERROR: couldn't mount RDWR because of unsupported optional features (1).
Here the file system is unable to mount the volume in the RW mode. That means that that feature is
a RO Compat feature. Another look at the table and it becomes apparent that the volume had the
unwritten feature enabled.
In both cases, the user has the option of disabling the feature. In the second case, the user has
the choice of mounting the volume in the RO mode.
GETTING STARTED
The OCFS2 software is split into two components, namely, kernel and tools. The kernel component includes
the core file system and the cluster stack, and is packaged along with the kernel. The tools component is
packaged as ocfs2-tools and needs to be specifically installed. It provides utilities to format, tune,
mount, debug and check the file system.
To install ocfs2-tools, refer to the package handling utility in in your distributions.
The next step is selecting a cluster stack. The options include:
A. No cluster stack, or local mount.
B. In-kernel o2cb cluster stack with local or global heartbeat.
C. Userspace cluster stacks pcmk or cman.
The file system allows changing cluster stacks easily using tunefs.ocfs2(8). To list the cluster stacks
stamped on the OCFS2 volumes, do:
# mounted.ocfs2 -d
Device Stack Cluster F UUID Label
/dev/sdb1 o2cb webcluster G DCDA2845177F4D59A0F2DCD8DE507CC3 hbvol1
/dev/sdc1 None 23878C320CF3478095D1318CB5C99EED localmount
/dev/sdd1 o2cb webcluster G 8AB016CD59FC4327A2CDAB69F08518E3 webvol
/dev/sdg1 o2cb webcluster G 77D95EF51C0149D2823674FCC162CF8B logsvol
/dev/sdh1 o2cb webcluster G BBA1DBD0F73F449384CE75197D9B7098 scratch
NON-CLUSTERED OR LOCAL MOUNT
To format a OCFS2 volume as a non-clustered (local) volume, do:
# mkfs.ocfs2 -L "mylabel" --fs-features=local /dev/sda1
To convert an existing clustered volume to a non-clustered volume, do:
# tunefs.ocfs2 --fs-features=local /dev/sda1
Non-clustered volumes do not interact with the cluster stack. One can have both clustered and non-
clustered volumes mounted at the same time.
While formatting a non-clustered volume, users should consider the possibility of later converting
that volume to a clustered one. If there is a possibility of that, then the user should add enough
node-slots using the -N option. Adding node-slots during format creates journals with large
extents. If created later, then the journals will be fragmented which is not good for performance.
CLUSTERED MOUNT WITH O2CB CLUSTER STACK
Only one of the two heartbeat mode can be active at any one time. Changing heartbeat modes is an
offline operation.
Both heartbeat modes require /etc/ocfs2/cluster.conf and /etc/sysconfig/o2cb to be populated as
described in ocfs2.cluster.conf(5) and o2cb.sysconfig(5) respectively. The only difference in set
up between the two modes is that global requires heartbeat devices to be configured whereas local
does not.
Refer o2cb(7) for more information.
LOCAL HEARTBEAT
This is the default heartbeat mode. The user needs to populate the configuration files as
described in ocfs2.cluster.conf(5) and o2cb.sysconfig(5). In this mode, the cluster stack
heartbeats on all mounted volumes. Thus, one does not have to specify heartbeat devices in
cluster.conf.
Once configured, the o2cb cluster stack can be onlined and offlined as follows:
# service o2cb online
Setting cluster stack "o2cb": OK
Registering O2CB cluster "webcluster": OK
Setting O2CB cluster timeouts : OK
# service o2cb offline
Clean userdlm domains: OK
Stopping O2CB cluster webcluster: OK
Unregistering O2CB cluster "webcluster": OK
GLOBAL HEARTBEAT
The configuration is similar to local heartbeat. The one additional step in this mode is
that it requires heartbeat devices to be also configured.
These heartbeat devices are OCFS2 formatted volumes with global heartbeat enabled on disk.
These volumes can later be mounted and used as clustered file systems.
The steps to format a volume with global heartbeat enabled is listed in o2cb(7). Also
listed there is listing all volumes with the cluster stack stamped on disk.
In this mode, the heartbeat is started when the cluster is onlined and stopped when the
cluster is offlined.
# service o2cb online
Setting cluster stack "o2cb": OK
Registering O2CB cluster "webcluster": OK
Setting O2CB cluster timeouts : OK
Starting global heartbeat for cluster "webcluster": OK
# service o2cb offline
Clean userdlm domains: OK
Stopping global heartbeat on cluster "webcluster": OK
Stopping O2CB cluster webcluster: OK
Unregistering O2CB cluster "webcluster": OK
# service o2cb status
Driver for "configfs": Loaded
Filesystem "configfs": Mounted
Stack glue driver: Loaded
Stack plugin "o2cb": Loaded
Driver for "ocfs2_dlmfs": Loaded
Filesystem "ocfs2_dlmfs": Mounted
Checking O2CB cluster "webcluster": Online
Heartbeat dead threshold: 31
Network idle timeout: 30000
Network keepalive delay: 2000
Network reconnect delay: 2000
Heartbeat mode: Global
Checking O2CB heartbeat: Active
77D95EF51C0149D2823674FCC162CF8B /dev/sdg1
Nodes in O2CB cluster: 92 96
CLUSTERED MOUNT WITH USERSPACE CLUSTER STACK
Configure and online the userspace stack pcmk or cman before using tunefs.ocfs2(8) to update the
cluster stack on disk.
# tunefs.ocfs2 --update-cluster-stack /dev/sdd1
Updating on-disk cluster information to match the running cluster.
DANGER: YOU MUST BE ABSOLUTELY SURE THAT NO OTHER NODE IS USING THIS
FILESYSTEM BEFORE MODIFYING ITS CLUSTER CONFIGURATION.
Update the on-disk cluster information? y
Refer to the cluster stack documentation for information on starting and stopping the cluster
stack.
FILE SYSTEM UTILITIES
This sections lists the utilities that are used to manage the OCFS2 file systems. This includes tools to
format, tune, check, mount, debug the file system. Each utility has a man page that lists its
capabilities in detail.
mkfs.ocfs2(8)
This is the file system format utility. All volumes have to be formatted prior to its use. As
this utility overwrites the volume, use it with care. Double check to ensure the volume is not in
use on any node in the cluster.
As a precaution, the utility will abort if the volume is locally mounted. It also detects use
across the cluster if used by OCFS2. But these checks are not comprehensive and can be overridden.
So use it with care.
While it is not always required, the cluster should be online.
tunefs.ocfs2(8)
This is the file system tune utility. It allows users to change certain on-disk parameters like
label, uuid, number of node-slots, volume size and the size of the journals. It also allows
turning on and off the file system features as listed above.
This utility requires the cluster to be online.
fsck.ocfs2(8)
This is the file system check utility. It detects and fixes on-disk errors. All the check codes
and their fixes are listed in fsck.ocfs2.checks(8).
This utility requires the cluster to be online to ensure the volume is not in use on another node
and to prevent the volume from being mounted for the duration of the check.
mount.ocfs2(8)
This is the file system mount utility. It is invoked indirectly by the mount(8) utility.
This utility detects the cluster status and aborts if the cluster is offline or does not match the
cluster stamped on disk.
o2cluster(8)
This is the file system cluster stack update utility. It allows the users to update the on-disk
cluster stack to the one provided.
This utility only updates the disk if the utility is reasonably assured that the file system is
not in use on any node.
o2info(1)
This is the file system information utility. It provides information like the features enabled on
disk, block size, cluster size, free space fragmentation, etc.
It can be used by both privileged and non-privileged users. Users having read permission on the
device can provide the path to the device. Other users can provide the path to a file on a mounted
file system.
debugfs.ocfs2(8)
This is the file system debug utility. It allows users to examine all file system structures
including walking directory structures, displaying inodes, backing up files, etc., without
mounting the file system.
This utility requires the user to have read permission on the device.
o2image(8)
This is the file system image utility. It allows users to copy the file system metadata skeleton,
including the inodes, directories, bitmaps, etc. As it excludes data, it shrinks the size of the
file system tremendously.
The image file created can be used in debugging on-disk corruptions.
mounted.ocfs2(8)
This is the file system detect utility. It detects all OCFS2 volumes in the system and lists its
label, uuid and cluster stack.
O2CB CLUSTER STACK UTILITIES
This sections lists the utilities that are used to manage O2CB cluster stack. Each utility has a man
page that lists its capabilities in detail.
o2cb(8)
This is the cluster configuration utility. It allows users to update the cluster configuration by
adding and removing nodes and heartbeat regions. This utility is used by the o2cb init script to
online and offline the cluster.
This is a new utility and replaces o2cb_ctl(8) which has been deprecated.
ocfs2_hb_ctl(8)
This is the cluster heartbeat utility. It allows users to start and stop local heartbeat. This
utility is invoked by mount.ocfs2(8) and should not be invoked directly by the user.
o2hbmonitor(8)
This is the disk heartbeat monitor. It tracks the elapsed time since the last heartbeat and logs
warnings once that time exceeds the warn threshold.
FILE SYSTEM NOTES
This section includes some useful notes that may prove helpful to the user.
BALANCED CLUSTER
A cluster is a computer. This is a fact and not a slogan. What this means is that an errant node
in the cluster can affect the behavior of other nodes. If one node is slow, the cluster operations
will slow down on all nodes. To prevent that, it is best to have a balanced cluster. This is a
cluster that has equally powered and loaded nodes.
The standard recommendation for such clusters is to have identical hardware and software across
all the nodes. However, that is not a hard and fast rule. After all, we have taken the effort to
ensure that OCFS2 works in a mixed architecture environment.
If one uses OCFS2 in a mixed architecture environment, try to ensure that the nodes are equally
powered and loaded. The use of a load balancer can assist with the latter. Power refers to the
number of processors, speed, amount of memory, I/O throughput, network bandwidth, etc. In reality,
having equally powered heterogeneous nodes is not always practical. In that case, make the lower
node numbers more powerful than the higher node numbers. The O2CB cluster stack favors lower node
numbers in all of its tiebreaking logic.
This is not to suggest you should add a single core node in a cluster of quad cores. No amount of
node number juggling will help you there.
FILE DELETION
In Linux, rm(1) removes the directory entry. It does not necessarily delete the corresponding
inode. But by removing the directory entry, it gives the illusion that the inode has been deleted.
This puzzles users when they do not see a corresponding up-tick in the reported free space. The
reason is that inode deletion has a few more hurdles to cross.
First is the hard link count, that indicates the number of directory entries pointing to that
inode. As long as an inode has one or more directory entries pointing to it, it cannot be deleted.
The file system has to wait for the removal of all those directory entries. In other words, wait
for that count to drop to zero.
The second hurdle is the POSIX semantics allowing files to be unlinked even while they are in-use.
In OCFS2, that translates to in-use across the cluster. The file system has to wait for all
processes across the cluster to stop using the inode.
Once these conditions are met, the inode is deleted and the freed space is visible after the next
sync.
Now the amount of space freed depends on the allocation. Only space that is actually allocated to
that inode is freed. The example below shows a sparsely allocated file of size 51TB of which only
2.4GB is actually allocated.
$ ls -lsh largefile
2.4G -rw-r--r-- 1 mark mark 51T Sep 29 15:04 largefile
Furthermore, for reflinked files, only private extents are freed. Shared extents are freed when
the last inode accessing it, is deleted. The example below shows a 4GB file that shares 3GB with
other reflinked files. Deleting it will increase the free space by 1GB. However, if it is the only
remaining file accessing the shared extents, the full 4G will be freed. (More information on the
shared-du(1) utility is provided below.)
$ shared-du -m -c --shared-size reflinkedfile
4000 (3000) reflinkedfile
The deletion itself is a multi-step process. Once the hard link count falls to zero, the inode is
moved to the orphan_dir system directory where it remains until the last process, across the
cluster, stops using the inode. Then the file system frees the extents and adds the freed space
count to the truncate_log system file where it remains until the next sync. The freed space is
made visible to the user only after that sync.
DIRECTORY LISTING
ls(1) may be a simple command, but it is not cheap. What is expensive is not the part where it
reads the directory listing, but the second part where it reads all the inodes, also referred as
an inode stat(2). If the inodes are not in cache, this can entail disk I/O. Now, while a cold
cache inode stat(2) is expensive in all file systems, it is especially so in a clustered file
system as it needs to take a cluster lock on each inode.
A hot cache stat(2), on the other hand, has shown to perform on OCFS2 like it does on EXT3.
In other words, the second ls(1) will be quicker than the first. However, it is not guaranteed.
Say you have a million files in a file system and not enough kernel memory to cache all the
inodes. In that case, each ls(1) will involve some cold cache stat(2)s.
ALLOCATION RESERVATION
Allocation reservation allows multiple concurrently extending files to grow as contiguously as
possible. One way to demonstrate its functioning is to run a script that extends multiple files in
a circular order. The script below does that by writing one hundred 4KB chunks to four files, one
after another.
$ for i in $(seq 0 99);
> do
> for j in $(seq 4);
> do
> dd if=/dev/zero of=file$j bs=4K count=1 seek=$i;
> done;
> done;
When run on a system running Linux kernel 2.6.34 or earlier, we end up with files with 100 extents
each. That is full fragmentation. As the files are being extended one after another, the on-disk
allocations are fully interleaved.
$ filefrag file1 file2 file3 file4
file1: 100 extents found
file2: 100 extents found
file3: 100 extents found
file4: 100 extents found
When run on a system running Linux kernel 2.6.35 or later, we see files with 7 extents each. That
is a lot fewer than before. Fewer extents mean more on-disk contiguity and that always leads to
better overall performance.
$ filefrag file1 file2 file3 file4
file1: 7 extents found
file2: 7 extents found
file3: 7 extents found
file4: 7 extents found
REFLINK OPERATION
This feature allows a user to create a writeable snapshot of a regular file. In this operation,
the file system creates a new inode with the same extent pointers as the original inode. Multiple
inodes are thus able to share data extents. This adds a twist in file system administration
because none of the existing file system utilities in Linux expect this behavior. du(1), a utility
to used to compute file space usage, simply adds the blocks allocated to each inode. As it does
not know about shared extents, it over estimates the space used. Say, we have a 5GB file in a
volume having 42GB free.
$ ls -l
total 5120000
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:15 myfile
$ du -m myfile*
5000 myfile
$ df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/sdd1 50G 8.2G 42G 17% /ocfs2
If we were to reflink it 4 times, we would expect the directory listing to report five 5GB files,
but the df(1) to report no loss of available space. du(1), on the other hand, would report the
disk usage to climb to 25GB.
$ reflink myfile myfile-ref1
$ reflink myfile myfile-ref2
$ reflink myfile myfile-ref3
$ reflink myfile myfile-ref4
$ ls -l
total 25600000
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:15 myfile
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:16 myfile-ref1
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:16 myfile-ref2
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:16 myfile-ref3
-rw-r--r-- 1 jeff jeff 5242880000 Sep 24 17:16 myfile-ref4
$ df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/sdd1 50G 8.2G 42G 17% /ocfs2
$ du -m myfile*
5000 myfile
5000 myfile-ref1
5000 myfile-ref2
5000 myfile-ref3
5000 myfile-ref4
25000 total
Enter shared-du(1), a shared extent-aware du. This utility reports the shared extents per file in
parenthesis and the overall footprint. As expected, it lists the overall footprint at 5GB. One can
view the details of the extents using shared-filefrag(1). Both these utilities are available at
http://oss.oracle.com/~smushran/reflink-tools/. We are currently in the process of pushing the
changes to the upstream maintainers of these utilities.
$ shared-du -m -c --shared-size myfile*
5000 (5000) myfile
5000 (5000) myfile-ref1
5000 (5000) myfile-ref2
5000 (5000) myfile-ref3
5000 (5000) myfile-ref4
25000 total
5000 footprint
# shared-filefrag -v myfile
Filesystem type is: 7461636f
File size of myfile is 5242880000 (1280000 blocks, blocksize 4096)
ext logical physical expected length flags
0 0 2247937 8448
1 8448 2257921 2256384 30720
2 39168 2290177 2288640 30720
3 69888 2322433 2320896 30720
4 100608 2354689 2353152 30720
7 192768 2451457 2449920 30720
. . .
37 1073408 2032129 2030592 30720 shared
38 1104128 2064385 2062848 30720 shared
39 1134848 2096641 2095104 30720 shared
40 1165568 2128897 2127360 30720 shared
41 1196288 2161153 2159616 30720 shared
42 1227008 2193409 2191872 30720 shared
43 1257728 2225665 2224128 22272 shared,eof
myfile: 44 extents found
DATA COHERENCY
One of the challenges in a shared file system is data coherency when multiple nodes are writing to
the same set of files. NFS, for example, provides close-to-open data coherency that results in the
data being flushed to the server when the file is closed on the client. This leaves open a wide
window for stale data being read on another node.
A simple test to check the data coherency of a shared file system involves concurrently appending
the same file. Like running "uname -a >>/dir/file" using a parallel distributed shell like dsh or
pconsole. If coherent, the file will contain the results from all nodes.
# dsh -R ssh -w node32,node33,node34,node35 "uname -a >> /ocfs2/test"
# cat /ocfs2/test
Linux node32 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
Linux node35 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
Linux node33 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
Linux node34 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
OCFS2 is a fully cache coherent cluster file system.
DISCONTIGUOUS BLOCK GROUP
Most file systems pre-allocate space for inodes during format. OCFS2 dynamically allocates this
space when required.
However, this dynamic allocation has been problematic when the free space is very fragmented,
because the file system required the inode and extent allocators to grow in contiguous fixed-size
chunks.
The discontiguous block group feature takes care of this problem by allowing the allocators to
grow in smaller, variable-sized chunks.
This feature was added in Linux kernel 2.6.35 and requires enabling on-disk feature discontig-bg.
BACKUP SUPER BLOCKS
A file system super block stores critical information that is hard to recreate. In OCFS2, it
stores the block size, cluster size, and the locations of the root and system directories, among
other things. As this block is close to the start of the disk, it is very susceptible to being
overwritten by an errant write. Say, dd if=file of=/dev/sda1.
Backup super blocks are copies of the super block. These blocks are dispersed in the volume to
minimize the chances of being overwritten. On the small chance that the original gets corrupted,
the backups are available to scan and fix the corruption.
mkfs.ocfs2(8) enables this feature by default. Users can disable this by specifying --fs-
features=nobackup-super during format.
o2info(1) can be used to view whether the feature has been enabled on a device.
# o2info --fs-features /dev/sdb1
backup-super strict-journal-super sparse extended-slotmap inline-data xattr
indexed-dirs refcount discontig-bg clusterinfo unwritten
In OCFS2, the super block is on the third block. The backups are located at the 1G, 4G, 16G, 64G,
256G and 1T byte offsets. The actual number of backup blocks depends on the size of the device.
The super block is not backed up on devices smaller than 1GB.
fsck.ocfs2(8) refers to these six offsets by numbers, 1 to 6. Users can specify any backup with
the -r option to recover the volume. The example below uses the second backup. If successful,
fsck.ocfs2(8) overwrites the corrupted super block with the backup.
# fsck.ocfs2 -f -r 2 /dev/sdb1
fsck.ocfs2 1.8.0
[RECOVER_BACKUP_SUPERBLOCK] Recover superblock information from backup block#1048576? <n> y
Checking OCFS2 filesystem in /dev/sdb1:
Label: webhome
UUID: B3E021A2A12B4D0EB08E9E986CDC7947
Number of blocks: 13107196
Block size: 4096
Number of clusters: 13107196
Cluster size: 4096
Number of slots: 8
/dev/sdb1 was run with -f, check forced.
Pass 0a: Checking cluster allocation chains
Pass 0b: Checking inode allocation chains
Pass 0c: Checking extent block allocation chains
Pass 1: Checking inodes and blocks.
Pass 2: Checking directory entries.
Pass 3: Checking directory connectivity.
Pass 4a: checking for orphaned inodes
Pass 4b: Checking inodes link counts.
All passes succeeded.
SYNTHETIC FILE SYSTEMS
The OCFS2 development effort included two synthetic file systems, configfs and dlmfs. It also
makes use of a third, debugfs.
configfs
configfs has since been accepted as a generic kernel component and is also used by
netconsole and fs/dlm. OCFS2 tools use it to communicate the list of nodes in the cluster,
details of the heartbeat device, cluster timeouts, and so on to the in-kernel node manager.
The o2cb init script mounts this file system at /sys/kernel/config.
dlmfs dlmfs exposes the in-kernel o2dlm to the user-space. While it was developed primarily for
OCFS2 tools, it has seen usage by others looking to add a cluster locking dimension in
their applications. Users interested in doing the same should look at the libo2dlm library
provided by ocfs2-tools. The o2cb init script mounts this file system at /dlm.
debugfs
OCFS2 uses debugfs to expose its in-kernel information to user space. For example, listing
the file system cluster locks, dlm locks, dlm state, o2net state, etc. Users can access the
information by mounting the file system at /sys/kernel/debug. To automount, add the
following to /etc/fstab: debugfs /sys/kernel/debug debugfs defaults 0 0
DISTRIBUTED LOCK MANAGER
One of the key technologies in a cluster is the lock manager, which maintains the locking state of
all resources across the cluster. An easy implementation of a lock manager involves designating
one node to handle everything. In this model, if a node wanted to acquire a lock, it would send
the request to the lock manager. However, this model has a weakness: lock manager’s death causes
the cluster to seize up.
A better model is one where all nodes manage a subset of the lock resources. Each node maintains
enough information for all the lock resources it is interested in. On event of a node death, the
remaining nodes pool in the information to reconstruct the lock state maintained by the dead node.
In this scheme, the locking overhead is distributed amongst all the nodes. Hence, the term
distributed lock manager.
O2DLM is a distributed lock manager. It is based on the specification titled "Programming Locking
Application" written by Kristin Thomas and is available at the following link.
http://opendlm.sourceforge.net/cvsmirror/opendlm/docs/dlmbook_final.pdf
DLM DEBUGGING
O2DLM has a rich debugging infrastructure that allows it to show the state of the lock manager,
all the lock resources, among other things. The figure below shows the dlm state of a nine-node
cluster that has just lost three nodes: 12, 32, and 35. It can be ascertained that node 7, the
recovery master, is currently recovering node 12 and has received the lock states of the dead node
from all other live nodes.
# cat /sys/kernel/debug/o2dlm/45F81E3B6F2B48CCAAD1AE7945AB2001/dlm_state
Domain: 45F81E3B6F2B48CCAAD1AE7945AB2001 Key: 0x10748e61
Thread Pid: 24542 Node: 7 State: JOINED
Number of Joins: 1 Joining Node: 255
Domain Map: 7 31 33 34 40 50
Live Map: 7 31 33 34 40 50
Lock Resources: 48850 (439879)
MLEs: 0 (1428625)
Blocking: 0 (1066000)
Mastery: 0 (362625)
Migration: 0 (0)
Lists: Dirty=Empty Purge=Empty PendingASTs=Empty PendingBASTs=Empty
Purge Count: 0 Refs: 1
Dead Node: 12
Recovery Pid: 24543 Master: 7 State: ACTIVE
Recovery Map: 12 32 35
Recovery Node State:
7 - DONE
31 - DONE
33 - DONE
34 - DONE
40 - DONE
50 - DONE
The figure below shows the state of a dlm lock resource that is mastered (owned) by node 25, with
6 locks in the granted queue and node 26 holding the EX (writelock) lock on that resource.
# debugfs.ocfs2 -R "dlm_locks M000000000000000022d63c00000000" /dev/sda1
Lockres: M000000000000000022d63c00000000 Owner: 25 State: 0x0
Last Used: 0 ASTs Reserved: 0 Inflight: 0 Migration Pending: No
Refs: 8 Locks: 6 On Lists: None
Reference Map: 26 27 28 94 95
Lock-Queue Node Level Conv Cookie Refs AST BAST Pending-Action
Granted 94 NL -1 94:3169409 2 No No None
Granted 28 NL -1 28:3213591 2 No No None
Granted 27 NL -1 27:3216832 2 No No None
Granted 95 NL -1 95:3178429 2 No No None
Granted 25 NL -1 25:3513994 2 No No None
Granted 26 EX -1 26:3512906 2 No No None
The figure below shows a lock from the file system perspective. Specifically, it shows a lock that
is in the process of being upconverted from a NL to EX. Locks in this state are are referred to in
the file system as busy locks and can be listed using the debugfs.ocfs2 command, "fs_locks -B".
# debugfs.ocfs2 -R "fs_locks -B" /dev/sda1
Lockres: M000000000000000000000b9aba12ec Mode: No Lock
Flags: Initialized Attached Busy
RO Holders: 0 EX Holders: 0
Pending Action: Convert Pending Unlock Action: None
Requested Mode: Exclusive Blocking Mode: No Lock
PR > Gets: 0 Fails: 0 Waits Total: 0us Max: 0us Avg: 0ns
EX > Gets: 1 Fails: 0 Waits Total: 544us Max: 544us Avg: 544185ns
Disk Refreshes: 1
With this debugging infrastructure in place, users can debug hang issues as follows:
* Dump the busy fs locks for all the OCFS2 volumes on the node with hanging processes. If no
locks are found, then the problem is not related to O2DLM.
* Dump the corresponding dlm lock for all the busy fs locks. Note down the owner (master) of
all the locks.
* Dump the dlm locks on the master node for each lock.
At this stage, one should note that the hanging node is waiting to get an AST from the master. The
master, on the other hand, cannot send the AST until the current holder has down converted that
lock, which it will do upon receiving a Blocking AST. However, a node can only down convert if all
the lock holders have stopped using that lock. After dumping the dlm lock on the master node,
identify the current lock holder and dump both the dlm and fs locks on that node.
The trick here is to see whether the Blocking AST message has been relayed to file system. If not,
the problem is in the dlm layer. If it has, then the most common reason would be a lock holder,
the count for which is maintained in the fs lock.
At this stage, printing the list of process helps.
$ ps -e -o pid,stat,comm,wchan=WIDE-WCHAN-COLUMN
Make a note of all D state processes. At least one of them is responsible for the hang on the
first node.
The challenge then is to figure out why those processes are hanging. Failing that, at least get
enough information (like alt-sysrq t output) for the kernel developers to review. What to do next
depends on where the process is hanging. If it is waiting for the I/O to complete, the problem
could be anywhere in the I/O subsystem, from the block device layer through the drivers to the
disk array. If the hang concerns a user lock (flock(2)), the problem could be in the user’s
application. A possible solution could be to kill the holder. If the hang is due to tight or
fragmented memory, free up some memory by killing non-essential processes.
The thing to note is that the symptom for the problem was on one node but the cause is on another.
The issue can only be resolved on the node holding the lock. Sometimes, the best solution will be
to reset that node. Once killed, the O2DLM recovery process will clear all locks owned by the dead
node and let the cluster continue to operate. As harsh as that sounds, at times it is the only
solution. The good news is that, by following the trail, you now have enough information to file a
bug and get the real issue resolved.
NFS EXPORTING
OCFS2 volumes can be exported as NFS volumes. This support is limited to NFS version 3, which
translates to Linux kernel version 2.4 or later.
If the version of the Linux kernel on the system exporting the volume is older than 2.6.30, then
the NFS clients must mount the volumes using the nordirplus mount option. This disables the
READDIRPLUS RPC call to workaround a bug in NFSD, detailed in the following link:
http://oss.oracle.com/pipermail/ocfs2-announce/2008-June/000025.html
Users running NFS version 2 can export the volume after having disabled subtree checking (mount
option no_subtree_check). Be warned, disabling the check has security implications (documented in
the exports(5) man page) that users must evaluate on their own.
FILE SYSTEM LIMITS
OCFS2 has no intrinsic limit on the total number of files and directories in the file system. In
general, it is only limited by the size of the device. But there is one limit imposed by the
current filesystem. It can address at most four billion clusters. A file system with 1MB cluster
size can go up to 4PB, while a file system with a 4KB cluster size can address up to 16TB.
SYSTEM OBJECTS
The OCFS2 file system stores its internal meta-data, including bitmaps, journals, etc., as system
files. These are grouped in a system directory. These files and directories are not accessible via
the file system interface but can be viewed using the debugfs.ocfs2(8) tool.
To list the system directory (referred to as double-slash), do:
# debugfs.ocfs2 -R "ls -l //" /dev/sde1
66 drwxr-xr-x 10 0 0 3896 19-Jul-2011 13:36 .
66 drwxr-xr-x 10 0 0 3896 19-Jul-2011 13:36 ..
67 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 bad_blocks
68 -rw-r--r-- 1 0 0 1179648 19-Jul-2011 13:36 global_inode_alloc
69 -rw-r--r-- 1 0 0 4096 19-Jul-2011 14:35 slot_map
70 -rw-r--r-- 1 0 0 1048576 19-Jul-2011 13:36 heartbeat
71 -rw-r--r-- 1 0 0 53686960128 19-Jul-2011 13:36 global_bitmap
72 drwxr-xr-x 2 0 0 3896 25-Jul-2011 15:05 orphan_dir:0000
73 drwxr-xr-x 2 0 0 3896 19-Jul-2011 13:36 orphan_dir:0001
74 -rw-r--r-- 1 0 0 8388608 19-Jul-2011 13:36 extent_alloc:0000
75 -rw-r--r-- 1 0 0 8388608 19-Jul-2011 13:36 extent_alloc:0001
76 -rw-r--r-- 1 0 0 121634816 19-Jul-2011 13:36 inode_alloc:0000
77 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 inode_alloc:0001
77 -rw-r--r-- 1 0 0 268435456 19-Jul-2011 13:36 journal:0000
79 -rw-r--r-- 1 0 0 268435456 19-Jul-2011 13:37 journal:0001
80 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 local_alloc:0000
81 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 local_alloc:0001
82 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 truncate_log:0000
83 -rw-r--r-- 1 0 0 0 19-Jul-2011 13:36 truncate_log:0001
The file names that end with numbers are slot specific and are referred to as node-local system
files. The set of node-local files used by a node can be determined from the slot map. To list the
slot map, do:
# debugfs.ocfs2 -R "slotmap" /dev/sde1
Slot# Node#
0 32
1 35
2 40
3 31
4 34
5 33
For more information, refer to the OCFS2 support guides available in the Documentation section at
http://oss.oracle.com/projects/ocfs2.
HEARTBEAT, QUORUM, AND FENCING
Heartbeat is an essential component in any cluster. It is charged with accurately designating
nodes as dead or alive. A mistake here could lead to a cluster hang or a corruption.
o2hb is the disk heartbeat component of o2cb. It periodically updates a timestamp on disk,
indicating to others that this node is alive. It also reads all the timestamps to identify other
live nodes. Other cluster components, like o2dlm and o2net, use the o2hb service to get node up
and down events.
The quorum is the group of nodes in a cluster that is allowed to operate on the shared storage.
When there is a failure in the cluster, nodes may be split into groups that can communicate in
their groups and with the shared storage but not between groups. o2quo determines which group is
allowed to continue and initiates fencing of the other group(s).
Fencing is the act of forcefully removing a node from a cluster. A node with OCFS2 mounted will
fence itself when it realizes that it does not have quorum in a degraded cluster. It does this so
that other nodes won’t be stuck trying to access its resources.
o2cb uses a machine reset to fence. This is the quickest route for the node to rejoin the cluster.
PROCESSES
[o2net]
One per node. It is a work-queue thread started when the cluster is brought on-line and
stopped when it is off-lined. It handles network communication for all mounts. It gets the
list of active nodes from O2HB and sets up a TCP/IP communication channel with each live
node. It sends regular keep-alive packets to detect any interruption on the channels.
[user_dlm]
One per node. It is a work-queue thread started when dlmfs is loaded and stopped when it is
unloaded (dlmfs is a synthetic file system that allows user space processes to access the
in-kernel dlm).
[ocfs2_wq]
One per node. It is a work-queue thread started when the OCFS2 module is loaded and stopped
when it is unloaded. It is assigned background file system tasks that may take cluster
locks like flushing the truncate log, orphan directory recovery and local alloc recovery.
For example, orphan directory recovery runs in the background so that it does not affect
recovery time.
[o2hb-14C29A7392]
One per heartbeat device. It is a kernel thread started when the heartbeat region is
populated in configfs and stopped when it is removed. It writes every two seconds to a
block in the heartbeat region, indicating that this node is alive. It also reads the region
to maintain a map of live nodes. It notifies subscribers like o2net and o2dlm of any
changes in the live node map.
[ocfs2dc]
One per mount. It is a kernel thread started when a volume is mounted and stopped when it
is unmounted. It downgrades locks in response to blocking ASTs (BASTs) requested by other
nodes.
[jbd2/sdf1-97]
One per mount. It is part of JBD2, which OCFS2 uses for journaling.
[ocfs2cmt]
One per mount. It is a kernel thread started when a volume is mounted and stopped when it
is unmounted. It works with kjournald2.
[ocfs2rec]
It is started whenever a node has to be recovered. This thread performs file system
recovery by replaying the journal of the dead node. It is scheduled to run after dlm
recovery has completed.
[dlm_thread]
One per dlm domain. It is a kernel thread started when a dlm domain is created and stopped
when it is destroyed. This thread sends ASTs and blocking ASTs in response to lock level
convert requests. It also frees unused lock resources.
[dlm_reco_thread]
One per dlm domain. It is a kernel thread that handles dlm recovery when another node dies.
If this node is the dlm recovery master, it re-masters every lock resource owned by the
dead node.
[dlm_wq]
One per dlm domain. It is a work-queue thread that o2dlm uses to queue blocking tasks.
FUTURE WORK
File system development is a never ending cycle. Faster and larger disks, faster and more number
of processors, larger caches, etc. keep changing the sweet spot for performance forcing developers
to rethink long held beliefs. Add to that new use cases, which forces developers to be innovative
in providing solutions that melds seamlessly with existing semantics.
We are currently looking to add features like transparent compression, transparent encryption,
delayed allocation, multi-device support, etc. as well as work on improving performance on newer
generation machines.
If you are interested in contributing, email the development team at ocfs2-devel@oss.oracle.com.
ACKNOWLEDGEMENTS
The principal developers of the OCFS2 file system, its tools and the O2CB cluster stack, are Joel Becker,
Zach Brown, Mark Fasheh, Jan Kara, Kurt Hackel, Tao Ma, Sunil Mushran, Tiger Yang and Tristan Ye.
Other developers who have contributed to the file system via bug fixes, testing, etc. are Wim Coekaerts,
Srinivas Eeda, Coly Li, Jeff Mahoney, Marcos Matsunaga, Goldwyn Rodrigues, Manish Singh and Wengang Wang.
The members of the Linux Cluster community including Andrew Beekhof, Lars Marowsky-Bree, Fabio Massimo Di
Nitto and David Teigland.
The members of the Linux File system community including Christoph Hellwig and Chris Mason.
The corporations that have contributed resources for this project including Oracle, SUSE Labs, EMC,
Emulex, HP, IBM, Intel and Network Appliance.
SEE ALSO
debugfs.ocfs2(8) fsck.ocfs2(8) fsck.ocfs2.checks(8) mkfs.ocfs2(8) mount.ocfs2(8) mounted.ocfs2(8)
o2cluster(8) o2image(8) o2info(1) o2cb(7) o2cb(8) o2cb.sysconfig(5) o2hbmonitor(8) ocfs2.cluster.conf(5)
tunefs.ocfs2(8)
AUTHOR
Oracle Corporation
COPYRIGHT
Copyright © 2004, 2012 Oracle. All rights reserved.
Version 1.8.7 January 2012 OCFS2(7)