Ubuntu Manpage: btrfs-balance - balance block groups on a btrfs filesystem

NAME

       btrfs-balance - balance block groups on a btrfs filesystem

SYNOPSIS

       btrfs balance <subcommand> <args>

DESCRIPTION

The primary purpose of the balance feature is to spread block groups across all devices so they match
constraints defined by the respective profiles. See mkfs.btrfs(8) section PROFILES for more details. The
scope of the balancing process can be further tuned by use of filters that can select the block groups to
process. Balance works only on a mounted filesystem. Extent sharing is preserved and reflinks are not
broken. Files are not defragmented nor recompressed, file extents are preserved but the physical
location on devices will change.

The balance operation is cancellable by the user. The on-disk state of the filesystem is always
consistent so an unexpected interruption (e.g. system crash, reboot) does not corrupt the filesystem. The
progress of the balance operation is temporarily stored as an internal state and will be resumed upon
mount, unless the mount option skip_balance is specified.

WARNING:
Running balance without filters will take a lot of time as it basically move data/metadata from the
whole filesystem and needs to update all block pointers.

The filters can be used to perform following actions:

• convert block group profiles (filter convert)

• make block group usage more compact (filter usage)

• perform actions only on a given device (filters devid, drange)

The filters can be applied to a combination of block group types (data, metadata, system). Note that
changing only the system type needs the force option. Otherwise system gets automatically converted
whenever metadata profile is converted.

When metadata redundancy is reduced (e.g. from RAID1 to single) the force option is also required and it
is noted in system log.

NOTE:
The balance operation needs enough work space, i.e. space that is completely unused in the filesystem,
otherwise this may lead to ENOSPC reports. See the section ENOSPC for more details.

COMPATIBILITY

       NOTE:
          The balance subcommand also exists under  the  btrfs  filesystem  namespace.   This  still  works  for
          backward compatibility but is deprecated and should not be used any more.

       NOTE:
          A  short  syntax btrfs balance <path> works due to backward compatibility but is deprecated and should
          not be used any more. Use btrfs balance start command instead.

PERFORMANCE IMPLICATIONS

       Balancing operations are very IO intensive and can also be quite CPU intensive, impacting  other  ongoing
       filesystem  operations.  Typically  large  amounts  of data are copied from one location to another, with
       corresponding metadata updates.

       Depending upon the block group layout, it can also be seek heavy. Performance on  rotational  devices  is
       noticeably worse compared to SSDs or fast arrays.

SUBCOMMAND

       cancel <path>
              cancels a running or paused balance, the command will block and wait until the current block group
              being processed completes

              Since kernel 5.7 the response time of the cancellation is significantly improved, on older kernels
              it might take a long time until currently processed chunk is completely finished.

       pause <path>
              pause  running  balance  operation,  this  will  store  the state of the balance progress and used
              filters to the filesystem

       resume <path>
              resume interrupted balance, the balance status must be stored on the filesystem from previous run,
              e.g. after it was paused or forcibly interrupted and mounted again with skip_balance

       start [options] <path>
              start the balance operation according to the specified filters, without any filters the  data  and
              metadata from the whole filesystem are moved. The process runs in the foreground.

              NOTE:
                 The  balance  command without filters will basically move everything in the filesystem to a new
                 physical location on devices (i.e. it does not affect the logical properties  of  file  extents
                 like  offsets  within  files  and  extent  sharing).   The  run  time is potentially very long,
                 depending on the filesystem size. To prevent starting a full balance by accident, the  user  is
                 warned  and  has a few seconds to cancel the operation before it starts.  The warning and delay
                 can be skipped with --full-balance option.

              Please note that the filters must be written together with the -d,  -m  and  -s  options,  because
              they're optional and bare -d and -m also work and mean no filters.

              NOTE:
                 When the target profile for conversion filter is raid5 or raid6, there's a safety timeout of 10
                 seconds to warn users about the status of the feature

              Options

              -d[<filters>]
                     act on data block groups, see section FILTERS for details about filters

              -m[<filters>]
                     act on metadata chunks, see FILTERS for details about filters

              -s[<filters>]
                     act on system chunks (requires -f), see FILTERS for details about filters.

              -f     force  a  reduction  of  metadata  integrity, e.g. when going from raid1 to single, or skip
                     safety timeout when the target conversion profile is raid5 or raid6

              --background|--bg
                     run the balance operation asynchronously in the  background,  uses  fork(2)  to  start  the
                     process that calls the kernel ioctl

              --enqueue
                     wait if there's another exclusive operation running, otherwise continue

              -v     (deprecated) alias for global '-v' option

       status [-v] <path>
              Show status of running or paused balance.

              Options

              -v     (deprecated) alias for global -v option

FILTERS

From kernel 3.3 onwards, BTRFS balance can limit its action to a subset of the whole filesystem, and can
be used to change the replication configuration (e.g. convert data from single to RAID1).

Balance can be limited to a block group profile with the following options:

• -d for data block groups

• -m for metadata block groups (also implicitly applies to -s)

• -s for system block groups

The options have an optional parameter which means that the parameter must start right after the option
without a space (this is mandatory getopt syntax), like -dusage=10. Options for all block group types can
be specified in one command.

A filter has the following structure: filter[=params][,filter=...]

To combine multiple filters use ,, without spaces. Example: -dconvert=raid1,soft

BTRFS can have different profiles on a single device or the same profile on multiple device.

The main reason why you want to have different profiles for data and metadata is to provide additional
protection of the filesystem's metadata when devices fail, since a single sector of unrecoverable
metadata will break the filesystem, while a single sector of lost data can be trivially recovered by
deleting the broken file.

Before changing profiles, make sure there is enough unallocated space on existing drives to create new
metadata block groups (for filesystems over 50GiB, this is 1GB * (number_of_devices + 2)).

Default profiles on BTRFS are:

• data: single

•

metadata:

• single devices: dup

• multiple devices: raid1

The available filter types are:

Filter types
profiles=<profiles>
Balances only block groups with the given profiles. Parameters are a list of profile names
separated by | (pipe).

usage=<percent>, usage=<range>
Balances only block groups with usage under the given percentage. The value of 0 is allowed and
will clean up completely unused block groups, this should not require any new work space
allocated. You may want to use usage=0 in case balance is returning ENOSPC and your filesystem is
not too full.

The argument may be a single value or a range. The single value N means at most N percent used,
equivalent to ..N range syntax. Kernels prior to 4.4 accept only the single value format. The
minimum range boundary is inclusive, maximum is exclusive.

devid=<id>
Balances only block groups which have at least one chunk on the given device. To list devices with
ids use btrfs filesystem show.

drange=<range>
Balance only block groups which overlap with the given byte range on any device. Use in
conjunction with devid to filter on a specific device. The parameter is a range specified as
start..end.

vrange=<range>
Balance only block groups which overlap with the given byte range in the filesystem's internal
virtual address space. This is the address space that most reports from btrfs in the kernel log
use. The parameter is a range specified as start..end.

convert=<profile>
Convert each selected block group to the given profile name identified by parameters.

NOTE:
Starting with kernel 4.5, the data chunks can be converted to/from the DUP profile on a single
device.

Starting with kernel 4.6, all profiles can be converted to/from DUP on multi-device
filesystems.

WARNING:
Bad or missing device are not detected immediately during runtime and this depends on some
later event like failed write or failed transaction commit. If there's a known failing device,
or a device deleted by /sys/block/<dev>/device/delete interface, the device will be still
accessed and written to.

In such case, one should not convert to a profile with lower redundancy (e.g. from RAID1 to
SINGLE), as attempts to create new chunks on the new devices will cause various problems.

The proper action is to use btrfs replace or btrfs device remove to handle the failing/missing
device first. Then convert will work with all devices correctly.

limit=<number>, limit=<range>
Process only given number of chunks, after all filters are applied. This can be used to
specifically target a chunk in connection with other filters (drange, vrange) or just simply limit
the amount of work done by a single balance run.

The argument may be a single value or a range. The single value N means at most N chunks,
equivalent to ..N range syntax. Kernels prior to 4.4 accept only the single value format. The
range minimum and maximum are inclusive.

stripes=<range>
Balance only block groups which have the given number of stripes. The parameter is a range
specified as start..end. Makes sense for block group profiles that utilize striping, i.e.
RAID0/10/5/6. The range minimum and maximum are inclusive.

soft Takes no parameters. Only has meaning when converting between profiles, or When doing convert from
one profile to another and soft mode is on, chunks that already have the target profile are left
untouched. This is useful e.g. when half of the filesystem was converted earlier but got
cancelled.

The soft mode switch is (like every other filter) per-type. For example, this means that we can
convert metadata chunks the "hard" way while converting data chunks selectively with soft switch.

Profile names, used in profiles and convert are one of:

• raid0

• raid1

• raid1c3

• raid1c4

• raid10

• raid5

• raid6

• dup

• single

The mixed data/metadata profiles can be converted in the same way, but conversion between mixed and
non-mixed is not implemented. For the constraints of the profiles please refer to mkfs.btrfs(8) section
PROFILES.

ENOSPC

       The way balance operates, it usually needs to temporarily create a new block group and move the old  data
       there,  before  the old block group can be removed.  For that it needs the work space, otherwise it fails
       for ENOSPC reasons.  This is not the same ENOSPC as if the free space is exhausted. This  refers  to  the
       space  on  the  level  of  block  groups, which are bigger parts of the filesystem that contain many file
       extents.

       The free work space can be calculated from the output of the btrfs filesystem show command:

          Label: 'BTRFS'  uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
                  Total devices 2 FS bytes used 77.03GiB
                  devid    1 size 53.90GiB used 51.90GiB path /dev/sdc2
                  devid    2 size 53.90GiB used 51.90GiB path /dev/sde1

       size - used = free work space

       53.90GiB - 51.90GiB = 2.00GiB

       An example of a filter that does not require workspace is usage=0. This  will  scan  through  all  unused
       block  groups  of  a  given type and will reclaim the space. After that it might be possible to run other
       filters.

       CONVERSIONS ON MULTIPLE DEVICES

       Conversion to profiles based on striping (RAID0, RAID5/6) require the  work  space  on  each  device.  An
       interrupted balance may leave partially filled block groups that consume the work space.

EXAMPLES

A more comprehensive example when going from one to multiple devices, and back, can be found in section
TYPICAL USECASES of btrfs-device(8).

MAKING BLOCK GROUP LAYOUT MORE COMPACT
The layout of block groups is not normally visible; most tools report only summarized numbers of free or
used space, but there are still some hints provided.

Let's use the following real life example and start with the output:

$ btrfs filesystem df /path
Data, single: total=75.81GiB, used=64.44GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is about 85%. How can we can
interpret that:

• chunks are filled by 85% on average, i.e. the usage filter with anything smaller than 85 will likely
not affect anything

• in a more realistic scenario, the space is distributed unevenly, we can assume there are completely
used chunks and the remaining are partially filled

Compacting the layout could be used on both. In the former case it would spread data of a given chunk to
the others and removing it. Here we can estimate that roughly 850 MiB of data have to be moved (85% of a
1 GiB chunk).

In the latter case, targeting the partially used chunks will have to move less data and thus will be
faster. A typical filter command would look like:

# btrfs balance start -dusage=50 /path
Done, had to relocate 2 out of 97 chunks

$ btrfs filesystem df /path
Data, single: total=74.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

As you can see, the total amount of data is decreased by just 1 GiB, which is an expected result. Let's
see what will happen when we increase the estimated usage filter.

# btrfs balance start -dusage=85 /path
Done, had to relocate 13 out of 95 chunks

$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Now the used/total ratio is about 94% and we moved about 74G - 68G = 6G of data to the remaining block
groups, i.e. the 6GiB are now free of filesystem structures, and can be reused for new data or metadata
block groups.

We can do a similar exercise with the metadata block groups, but this should not typically be necessary,
unless the used/total ratio is really off. Here the ratio is roughly 50% but the difference as an
absolute number is "a few gigabytes", which can be considered normal for a workload with snapshots or
reflinks updated frequently.

# btrfs balance start -musage=50 /path
Done, had to relocate 4 out of 89 chunks

$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=14.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Just 1 GiB decrease, which possibly means there are block groups with good utilization. Making the
metadata layout more compact would in turn require updating more metadata structures, i.e. lots of IO. As
running out of metadata space is a more severe problem, it's not necessary to keep the utilization ratio
too high. For the purpose of this example, let's see the effects of further compaction:

# btrfs balance start -musage=70 /path
Done, had to relocate 13 out of 88 chunks

$ btrfs filesystem df .
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=11.97GiB, used=8.83GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
Normally the balance operation needs a work space, to temporarily move the data before the old block
groups gets removed. If there's no work space, it ends with no space left.

There's a special case when the block groups are completely unused, possibly left after removing lots of
files or deleting snapshots. Removing empty block groups is automatic since 3.18. The same can be
achieved manually with a notable exception that this operation does not require the work space. Thus it
can be used to reclaim unused block groups to make it available.

# btrfs balance start -dusage=0 /path

This should lead to decrease in the total numbers in the btrfs filesystem df output.

EXIT STATUS

       Unless  indicated  otherwise  below,  all  btrfs  balance  subcommands  return a zero exit status if they
       succeed, and non zero in case of failure.

       The pause, cancel, and resume subcommands exit with a  status  of  2  if  they  fail  because  a  balance
       operation was not running.

       The  status  subcommand  exits  with  a  status  of  0  if  a  balance operation is not running, 1 if the
       command-line usage is incorrect or a balance operation is still running, and 2 on other errors.

AVAILABILITY

       btrfs is part of btrfs-progs.  Please refer to the documentation at https://btrfs.readthedocs.io.