mirror of
https://github.com/kdave/btrfs-progs
synced 2024-12-24 23:22:27 +00:00
a0137082de
- use :file: and :command: - simplify manual page references - add more web links - typo fixes - more cross-references Signed-off-by: David Sterba <dsterba@suse.com>
64 lines
2.8 KiB
ReStructuredText
64 lines
2.8 KiB
ReStructuredText
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
|
|
:doc:`mkfs.btrfs` section :ref:`PROFILES<man-mkfs-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 :command:`btrfs filesystem` namespace.
|
|
This still works for backward compatibility but is deprecated and should not
|
|
be used any more.
|
|
|
|
.. note::
|
|
A short syntax :command:`btrfs balance <path>` works due to backward compatibility
|
|
but is deprecated and should not be used any more. Use :command:`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.
|