Ubuntu Manpage: lvmlockd — LVM locking daemon

Provided by: lvm2-lockd_2.03.27-1ubuntu1_amd64

NAME

       lvmlockd — LVM locking daemon

SYNOPSIS

       lvmlockd [options]

DESCRIPTION

       LVM commands use lvmlockd to coordinate access to shared storage.
       When LVM is used on devices shared by multiple hosts, locks will:
       • coordinate reading and writing of LVM metadata
       • validate caching of LVM metadata
       • prevent conflicting activation of logical volumes
       lvmlockd uses an external lock manager to perform basic locking.

       Lock manager (lock type) options are:
       • sanlock: places locks on disk within LVM storage.
       • dlm: uses network communication and a cluster manager.

OPTIONS

       -h|--help
              Show this help information.

       -V|--version
              Show version of lvmlockd.

       -T|--test
              Test mode, do not call lock manager.

       -f|--foreground
              Don't fork.

       -D|--daemon-debug
              Don't fork and print debugging to stdout.

       -p|--pid-file path
              Set path to the pid file.

       -s|--socket-path path
              Set path to the socket to listen on.

       --adopt-file path
              Set path to the adopt file.

       -S|--syslog-priority err|warning|debug
              Write log messages from this level up to syslog.

       -g|--gl-type sanlock|dlm
              Set global lock type to be sanlock or dlm.

       -i|--host-id num
              Set the local sanlock host id.

       -F|--host-id-file path
              A file containing the local sanlock host_id.

       -o|--sanlock-timeout seconds
              Override the default sanlock I/O timeout.

       -A|--adopt 0|1
              Enable (1) or disable (0) lock adoption.

USAGE

Initial set up
Setting up LVM to use lvmlockd and a shared VG for the first time includes some one time set up steps:

1. choose a lock manager
dlm
If dlm (or corosync) are already being used by other cluster software, then select dlm. dlm uses
corosync which requires additional configuration beyond the scope of this document. See corosync and dlm
documentation for instructions on configuration, set up and usage.

sanlock
Choose sanlock if dlm/corosync are not otherwise required. sanlock does not depend on any clustering
software or configuration.

2. configure hosts to use lvmlockd
On all hosts running lvmlockd, configure lvm.conf:
use_lvmlockd = 1

sanlock
Assign each host a unique host_id in the range 1-2000 by setting
/etc/lvm/lvmlocal.conf local/host_id

3. start lvmlockd
Start the lvmlockd daemon.
Use systemctl, a cluster resource agent, or run directly, e.g.
systemctl start lvmlockd

4. start lock manager
sanlock
Start the sanlock and wdmd daemons.
Use systemctl or run directly, e.g.
systemctl start wdmd sanlock

dlm
Start the dlm and corosync daemons.
Use systemctl, a cluster resource agent, or run directly, e.g.
systemctl start corosync dlm

5. create VG on shared devices
vgcreate --shared <vgname> <devices>

The shared option sets the VG lock type to sanlock or dlm depending on which lock manager is running.
LVM commands acquire locks from lvmlockd, and lvmlockd uses the chosen lock manager.

6. start VG on all hosts
vgchange --lockstart

Shared VGs must be started before they are used. Starting the VG performs lock manager initialization
that is necessary to begin using locks (i.e. creating and joining a lockspace). Starting the VG may
take some time, and until the start completes the VG may not be modified or activated.

7. create and activate LVs
Standard lvcreate and lvchange commands are used to create and activate LVs in a shared VG.

An LV activated exclusively on one host cannot be activated on another. When multiple hosts need to use
the same LV concurrently, the LV can be activated with a shared lock (see lvchange options -aey vs -asy.)
(Shared locks are disallowed for certain LV types that cannot be used from multiple hosts.)

Normal start up and shut down
After initial set up, start up and shut down include the following steps. They can be performed directly
or may be automated using systemd or a cluster resource manager/agents. When using lvmlockd.service,
daemon options can be set in /etc/sysconfig/lvmlockd as OPTIONS='-x1 -y2'.

• start lvmlockd
• start lock manager
• vgchange --lockstart
• activate LVs in shared VGs

The shut down sequence is the reverse:

• deactivate LVs in shared VGs
• vgchange --lockstop
• stop lock manager
• stop lvmlockd

TOPICS

Protecting VGs on shared devices
The following terms are used to describe the different ways of accessing VGs on shared devices.

shared VG

A shared VG exists on shared storage that is visible to multiple hosts. LVM acquires locks through
lvmlockd to coordinate access to shared VGs. A shared VG has lock_type "dlm" or "sanlock", which
specifies the lock manager lvmlockd will use.

When the lock manager for the lock type is not available (e.g. not started or failed), lvmlockd is unable
to acquire locks for LVM commands. In this situation, LVM commands are only allowed to read and display
the VG; changes and activation will fail.

local VG

A local VG is meant to be used by a single host. It has no lock type or lock type "none". A local VG
typically exists on local (non-shared) devices and cannot be used concurrently from different hosts.

If a local VG does exist on shared devices, it should be owned by a single host by having the system ID
set, see lvmsystemid(7). The host with a matching system ID can use the local VG and other hosts will
ignore it. A VG with no lock type and no system ID should be excluded from all but one host using
lvm.conf filters. Without any of these protections, a local VG on shared devices can be easily damaged
or destroyed.

clvm VG

A clvm VG (or clustered VG) is a VG on shared storage (like a shared VG) that requires clvmd for
clustering and locking. See below for converting a clvm/clustered VG to a shared VG.

Shared VGs from hosts not using lvmlockd
Hosts that do not use shared VGs will not be running lvmlockd. In this case, shared VGs that are still
visible to the host will be ignored (like foreign VGs, see lvmsystemid(7)).

The --shared option for reporting and display commands causes shared VGs to be displayed on a host not
using lvmlockd, like the --foreign option does for foreign VGs.

Creating the first sanlock VG
When use_lvmlockd is first enabled in lvm.conf, and before the first sanlock VG is created, no global
lock will exist. In this initial state, LVM commands try and fail to acquire the global lock, producing
a warning, and some commands are disallowed. Once the first sanlock VG is created, the global lock will
be available, and LVM will be fully operational.

When a new sanlock VG is created, its lockspace is automatically started on the host that creates it.
Other hosts need to run 'vgchange --lockstart' to start the new VG before they can use it.

Creating the first sanlock VG is not protected by locking, so it requires special attention. This is
because sanlock locks exist on storage within the VG, so they are not available until after the VG is
created. The first sanlock VG that is created will automatically contain the "global lock". Be aware of
the following special considerations:

• The first vgcreate command needs to be given the path to a device that has not yet been initialized
with pvcreate. The pvcreate initialization will be done by vgcreate. This is because the pvcreate
command requires the global lock, which will not be available until after the first sanlock VG is
created.

• Because the first sanlock VG will contain the global lock, this VG needs to be accessible to all hosts
that will use sanlock shared VGs. All hosts will need to use the global lock from the first sanlock
VG.

• The device and VG name used by the initial vgcreate will not be protected from concurrent use by
another vgcreate on another host.

See below for more information about managing the sanlock global lock.

Using shared VGs
In the 'vgs' command, shared VGs are indicated by "s" (for shared) in the sixth attr field, and by
"shared" in the "--options shared" report field. The specific lock type and lock args for a shared VG
can be displayed with 'vgs -o+locktype,lockargs'.

Shared VGs need to be "started" and "stopped", unlike other types of VGs. See the following section for
a full description of starting and stopping.

Removing a shared VG will fail if other hosts have the VG started. Run vgchange --lockstop <vgname> on
all other hosts before vgremove. (It may take several seconds before vgremove recognizes that all hosts
have stopped a sanlock VG.)

Starting and stopping VGs
Starting a shared VG (vgchange --lockstart) causes the lock manager to start (join) the lockspace for the
VG on the host where it is run. This makes locks for the VG available to LVM commands on the host.
Before a VG is started, only LVM commands that read/display the VG are allowed to continue without locks
(and with a warning).

Stopping a shared VG (vgchange --lockstop) causes the lock manager to stop (leave) the lockspace for the
VG on the host where it is run. This makes locks for the VG inaccessible to the host. A VG cannot be
stopped while it has active LVs.

When using the lock type sanlock, starting a VG can take a long time (potentially minutes if the host was
previously shut down without cleanly stopping the VG.)

A shared VG can be started after all the following are true:

• lvmlockd is running
• the lock manager is running
• the VG's devices are visible on the system

A shared VG can be stopped if all LVs are deactivated.

All shared VGs can be started/stopped using:
vgchange --lockstart
vgchange --lockstop

Individual VGs can be started/stopped using:
vgchange --lockstart <vgname> ...
vgchange --lockstop <vgname> ...

To make vgchange not wait for start to complete:
vgchange --lockstart --lockopt nowait ...

lvmlockd can be asked directly to stop all lockspaces:
lvmlockctl -S|--stop-lockspaces

To start only selected shared VGs, use the lvm.conf activation/lock_start_list. When defined, only VG
names in this list are started by vgchange. If the list is not defined (the default), all visible shared
VGs are started. To start only "vg1", use the following lvm.conf configuration:

activation {
lock_start_list = [ "vg1" ]
...
}

Internal command locking
To optimize the use of LVM with lvmlockd, be aware of the three kinds of locks and when they are used:

Global lock

The global lock is associated with global information, which is information not isolated to a single VG.
This includes:

• The global VG namespace.
• The set of orphan PVs and unused devices.
• The properties of orphan PVs, e.g. PV size.

The global lock is acquired in shared mode by commands that read this information, or in exclusive mode
by commands that change it. For example, the command 'vgs' acquires the global lock in shared mode
because it reports the list of all VG names, and the vgcreate command acquires the global lock in
exclusive mode because it creates a new VG name, and it takes a PV from the list of unused PVs.

When an LVM command is given a tag argument, or uses select, it must read all VGs to match the tag or
selection, which causes the global lock to be acquired.

VG lock

A VG lock is associated with each shared VG. The VG lock is acquired in shared mode to read the VG and
in exclusive mode to change the VG or activate LVs. This lock serializes access to a VG with all other
LVM commands accessing the VG from all hosts.

The command 'vgs <vgname>' does not acquire the global lock (it does not need the list of all VG names),
but will acquire the VG lock on each VG name argument.

LV lock

An LV lock is acquired before the LV is activated, and is released after the LV is deactivated. If the
LV lock cannot be acquired, the LV is not activated. (LV locks are persistent and remain in place when
the activation command is done. Global and VG locks are transient, and are held only while an LVM
command is running.)

lock retries

If a request for a global or VG lock fails due to a lock conflict with another host, lvmlockd
automatically retries for a short time before returning a failure to the LVM command. If those retries
are insufficient, the LVM command will retry the entire lock request a number of times specified by
global/lvmlockd_lock_retries before failing. If a request for an LV lock fails due to a lock conflict,
the command fails immediately.

Managing the global lock in sanlock VGs
The global lock exists in one of the sanlock VGs. The first sanlock VG created will contain the global
lock. Subsequent sanlock VGs will each contain a disabled global lock that can be enabled later if
necessary.

The VG containing the global lock must be visible to all hosts using sanlock VGs. For this reason, it
can be useful to create a small sanlock VG, visible to all hosts, and dedicated to just holding the
global lock. While not required, this strategy can help to avoid difficulty in the future if VGs are
moved or removed.

The vgcreate command typically acquires the global lock, but in the case of the first sanlock VG, there
will be no global lock to acquire until the first vgcreate is complete. So, creating the first sanlock
VG is a special case that skips the global lock.

vgcreate determines that it's creating the first sanlock VG when no other sanlock VGs are visible on the
system. It is possible that other sanlock VGs do exist, but are not visible when vgcreate checks for
them. In this case, vgcreate will create a new sanlock VG with the global lock enabled. When the
another VG containing a global lock appears, lvmlockd will then see more than one VG with a global lock
enabled. LVM commands will report that there are duplicate global locks.

If the situation arises where more than one sanlock VG contains a global lock, the global lock should be
manually disabled in all but one of them with the command:

lvmlockctl --gl-disable <vgname>

(The one VG with the global lock enabled must be visible to all hosts.)

An opposite problem can occur if the VG holding the global lock is removed. In this case, no global lock
will exist following the vgremove, and subsequent LVM commands will fail to acquire it. In this case,
the global lock needs to be manually enabled in one of the remaining sanlock VGs with the command:

lvmlockctl --gl-enable <vgname>

(Using a small sanlock VG dedicated to holding the global lock can avoid the case where the global lock
must be manually enabled after a vgremove.)

Internal lvmlock LV
A sanlock VG contains a hidden LV called "lvmlock" that holds the sanlock locks. vgreduce cannot yet
remove the PV holding the lvmlock LV. To remove this PV, change the VG lock type to "none", run
vgreduce, then change the VG lock type back to "sanlock". Similarly, pvmove cannot be used on a PV used
by the lvmlock LV.

To place the lvmlock LV on a specific device, create the VG with only that device, then use vgextend to
add other devices.

LV activation
In a shared VG, LV activation involves locking through lvmlockd, and the following values are possible
with lvchange/vgchange -a:

y|ey The command activates the LV in exclusive mode, allowing a single host to activate the LV. Before
activating the LV, the command uses lvmlockd to acquire an exclusive lock on the LV. If the lock
cannot be acquired, the LV is not activated and an error is reported. This would happen if the LV
is active on another host.

sy The command activates the LV in shared mode, allowing multiple hosts to activate the LV
concurrently. Before activating the LV, the command uses lvmlockd to acquire a shared lock on the
LV. If the lock cannot be acquired, the LV is not activated and an error is reported. This would
happen if the LV is active exclusively on another host. If the LV type prohibits shared access,
such as a snapshot, the command will report an error and fail. The shared mode is intended for a
multi-host/cluster application or file system. LV types that cannot be used concurrently from
multiple hosts include thin, cache, raid, mirror, and snapshot.

n The command deactivates the LV. After deactivating the LV, the command uses lvmlockd to release
the current lock on the LV.

Manually repairing a shared VG
Some failure conditions may not be repairable while the VG has a shared lock type. In these cases, it
may be possible to repair the VG by forcibly changing the lock type to "none". This is done by adding
"--lockopt force" to the normal command for changing the lock type: vgchange --locktype none VG. The VG
lockspace should first be stopped on all hosts, and be certain that no hosts are using the VG before this
is done.

Recover from lost PV holding sanlock locks
In a sanlock VG, the sanlock locks are held on the hidden "lvmlock" LV. If the PV holding this LV is
lost, a new lvmlock LV needs to be created. To do this, ensure no hosts are using the VG, then forcibly
change the lock type to "none" (see above). Then change the lock type back to "sanlock" with the normal
command for changing the lock type: vgchange --locktype sanlock VG. This recreates the internal lvmlock
LV with the necessary locks.

Locking system failures
lvmlockd failure

If lvmlockd fails or is killed while holding locks, the locks are orphaned in the lock manager. Orphaned
locks must be cleared or adopted before the associated resources can be accessed normally. If lock
adoption is enabled, lvmlockd keeps a record of locks in the adopt-file. A subsequent instance of
lvmlockd will then adopt locks orphaned by the previous instance. Adoption must be enabled in both
instances (--adopt|-A 1). Without adoption, the lock manager or host would require a reset to clear
orphaned lock state.

dlm/corosync failure

If dlm or corosync fail, the clustering system will fence the host using a method configured within the
dlm/corosync clustering environment.

LVM commands on other hosts will be blocked from acquiring any locks until the dlm/corosync recovery
process is complete.

sanlock lease storage failure

If the PV under a sanlock VG's lvmlock LV is disconnected, unresponsive or too slow, sanlock cannot renew
the lease for the VG's locks. After some time, the lease will expire, and locks that the host owns in
the VG can be acquired by other hosts. The VG must be forcibly deactivated on the host with the expiring
lease before other hosts can acquire its locks. This is necessary for data protection.

When the sanlock daemon detects that VG storage is lost and the VG lease is expiring, it runs the command
lvmlockctl --kill <vgname>. This command emits a syslog message stating that storage is lost for the VG,
and that LVs in the VG must be immediately deactivated.

If no LVs are active in the VG, then the VG lockspace will be removed, and errors will be reported when
trying to use the VG. Use the lvmlockctl --drop command to clear the stale lockspace from lvmlockd.

If the VG has active LVs, they must be quickly deactivated before the locks expire. After all LVs are
deactivated, run lvmlockctl --drop <vgname> to clear the expiring lockspace from lvmlockd.

If all LVs in the VG are not deactivated within about 40 seconds, sanlock uses wdmd and the local
watchdog to reset the host. The machine reset is effectively a severe form of "deactivating" LVs before
they can be activated on other hosts. The reset is considered a better alternative than having LVs used
by multiple hosts at once, which could easily damage or destroy their content.

sanlock lease storage failure automation

When the sanlock daemon detects that the lease storage is lost, it runs the command lvmlockctl --kill
<vgname>. This lvmlockctl command can be configured to run another command to forcibly deactivate LVs,
taking the place of the manual process described above. The other command is configured in the lvm.conf
lvmlockctl_kill_command setting. The VG name is appended to the end of the command specified.

The lvmlockctl_kill_command should forcibly deactivate LVs in the VG, ensuring that existing writes to
LVs in the VG are complete and that further writes to the LVs in the VG will be rejected. If it is able
to do this successfully, it should exit with success, otherwise it should exit with an error. If
lvmlockctl --kill gets a successful result from lvmlockctl_kill_command, it tells lvmlockd to drop locks
for the VG (the equivalent of running lvmlockctl --drop). If this completes in time, a machine reset can
be avoided.

One possible option is to create a script my_vg_kill_script.sh:
#!/bin/bash
VG=$1
# replace dm table with the error target for top level LVs
dmsetup wipe_table -S "uuid=~LVM && vgname=$VG && lv_layer=\"\""
# check that the error target is in place
dmsetup table -c -S "uuid=~LVM && vgname=$VG && lv_layer=\"\"" |grep -vw error
if [[ $? -ne 0 ]] ; then
exit 0
fi
exit 1

Set in lvm.conf:
lvmlockctl_kill_command="/usr/sbin/my_vg_kill_script.sh"

(The script and dmsetup commands should be tested with the actual VG to ensure that all top level LVs are
properly disabled.)

If the lvmlockctl_kill_command is not configured, or fails, lvmlockctl --kill will emit syslog messages
as described in the previous section, notifying the user to manually deactivate the VG before sanlock
resets the machine.

sanlock daemon failure

If the sanlock daemon fails or exits while a lockspace is started, the local watchdog will reset the
host. This is necessary to protect any application resources that depend on sanlock leases.

Changing dlm cluster name
When a dlm VG is created, the cluster name is saved in the VG metadata. To use the VG, a host must be in
the named dlm cluster. If the dlm cluster name changes, or the VG is moved to a new cluster, the dlm
cluster name saved in the VG must also be changed.

To see the dlm cluster name saved in the VG, use the command:
vgs -o+locktype,lockargs <vgname>

To change the dlm cluster name in the VG when the VG is still used by the original cluster:

• Start the VG on the host changing the lock type
vgchange --lockstart <vgname>

• Stop the VG on all other hosts:
vgchange --lockstop <vgname>

• Change the VG lock type to none on the host where the VG is started:
vgchange --locktype none <vgname>

• Change the dlm cluster name on the hosts or move the VG to the new cluster. The new dlm cluster must
now be running on the host. Verify the new name by:
cat /sys/kernel/config/dlm/cluster/cluster_name

• Change the VG lock type back to dlm which sets the new cluster name:
vgchange --locktype dlm <vgname>

• Start the VG on hosts to use it:
vgchange --lockstart <vgname>

To change the dlm cluster name in the VG when the dlm cluster name has already been changed on the hosts,
or the VG has already moved to a different cluster:

• Ensure the VG is not being used by any hosts.

• The new dlm cluster must be running on the host making the change. The current dlm cluster name can be
seen by:
cat /sys/kernel/config/dlm/cluster/cluster_name

• Change the VG lock type to none:
vgchange --locktype none --lockopt force <vgname>

• Change the VG lock type back to dlm which sets the new cluster name:
vgchange --locktype dlm <vgname>

• Start the VG on hosts to use it:
vgchange --lockstart <vgname>

Changing a local VG to a shared VG
All LVs must be inactive to change the lock type.

lvmlockd must be configured and running as described in USAGE.

• Change a local VG to a shared VG with the command:
vgchange --locktype sanlock|dlm <vgname>

• Start the VG on hosts to use it:
vgchange --lockstart <vgname>

If lvmlockd or the cluster manager are not available, the lock type can be forcibly changed with:
vgchange --locktype sanlock|dlm --lockopt force <vgname>

Changing a shared VG to a local VG
All LVs must be inactive to change the lock type.

• Start the VG on the host making the change:
vgchange --lockstart <vgname>

• Stop the VG on all other hosts:
vgchange --lockstop <vgname>

• Change the VG lock type to none on the host where the VG is started:
vgchange --locktype none <vgname>

If the VG cannot be started with the previous lock type, then the lock type can be forcibly changed to
none with:
vgchange --locktype none --lockopt force <vgname>

To change a VG from one lock type to another (i.e. between sanlock and dlm), first change it to a local
VG, then to the new type.

Changing a clvm/clustered VG to a shared VG
All LVs must be inactive to change the lock type.

First change the clvm/clustered VG to a local VG. Within a running clvm cluster, change a clustered VG
to a local VG with the command:

vgchange -cn <vgname>

If the clvm cluster is no longer running on any nodes, then extra options can be used to forcibly make
the VG local. Caution: this is only safe if all nodes have stopped using the VG:

vgchange --locktype none --lockopt force <vgname>

After the VG is local, follow the steps described in "changing a local VG to a shared VG".

Extending an LV active on multiple hosts
With lvmlockd and dlm, a special clustering procedure is used to refresh a shared LV on remote cluster
nodes after it has been extended on one node.

When an LV holding gfs2 or ocfs2 is active on multiple hosts with a shared lock, lvextend is permitted to
run with an existing shared LV lock in place of the normal exclusive LV lock.

After lvextend has finished extending the LV, it sends a remote request to other nodes running the dlm to
run 'lvchange --refresh' on the LV. This uses dlm_controld and corosync features.

Some special --lockopt values can be used to modify this process. "shupdate" permits the lvextend update
with an existing shared lock if it isn't otherwise permitted. "norefresh" prevents the remote refresh
operation.

Limitations of shared VGs
Commands that cannot be run on shared VGs:
• splitting snapshots from LVs
• splitting mirrors in sanlock VGs
• pvmove of entire PVs, or under LVs activated with shared locks
• vgmerge

lvmlockd changes from clvmd
(See above for converting an existing clvm VG to a shared VG.)

While lvmlockd and clvmd are entirely different systems, LVM command usage remains similar. Differences
are more notable when using lvmlockd's sanlock option.

Visible usage differences between shared VGs (using lvmlockd) and clvm/clustered VGs (using clvmd):

• lvm.conf is configured to use lvmlockd by setting use_lvmlockd=1. clvmd used locking_type=3.

• vgcreate --shared creates a shared VG. vgcreate --clustered y created a clvm/clustered VG.

• lvmlockd adds the option of using sanlock for locking, avoiding the need for network clustering.

• lvmlockd defaults to the exclusive activation mode whenever the activation mode is unspecified, i.e.
-ay means -aey, not -asy.

• lvmlockd commands always apply to the local host, and never have an effect on a remote host. (The
activation option 'l' is not used.)

• lvmlockd saves the cluster name for a shared VG using dlm. Only hosts in the matching cluster can use
the VG.

• lvmlockd requires starting/stopping shared VGs with vgchange --lockstart and --lockstop.

• vgremove of a sanlock VG may fail indicating that all hosts have not stopped the VG lockspace. Stop
the VG on all hosts using vgchange --lockstop.

• vgreduce or pvmove of a PV in a sanlock VG will fail if it holds the internal "lvmlock" LV that holds
the sanlock locks.

• lvmlockd uses lock retries instead of lock queueing, so high lock contention may require increasing
global/lvmlockd_lock_retries to avoid transient lock failures.

• lvmlockd includes VG reporting options lock_type and lock_args, and LV reporting option lock_args to
view the corresponding metadata fields.

• In the 'vgs' command's sixth VG attr field, "s" for "shared" is displayed for shared VGs.

• If lvmlockd fails or is killed while in use, locks it held remain but are orphaned in the lock manager.
lvmlockd can be restarted with an option to adopt the orphan locks from the previous instance of
lvmlockd.

• The 'lvs' command does not report any remote state, because lvmlockd is unable to passively check the
remote active or lock state of an LV.

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

USAGE

TOPICS

SEE ALSO