Balance

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.

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.

Note

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

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.

Examples

Adding new device

The unallocated space requirements depend on the selected storage profiles. The requirements for the storage profile must be met for the selected for both data and metadata (e.g. if you have single data and RAID1 metadata, the stricter RAID1 requirements must be met or the filesystem may run out of metadata space and go read-only).

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

If using a striped profile (raid0, raid10, raid5, or raid6), then do a full data balance of all data after adding a drive. If adding multiple drives at once, do a full data balance after adding the last one.

btrfs balance start -v --full-balance mnt/

If the balance is interrupted, it can be restarted using the stripes filter (i.e. -dstripes=1..N where N is the previous size of the array before the new device was added) as long as all devices are the same size. If the device sizes are different, a specialized userspace balance tool is required. The data balance must be completed before adding any new devices or increasing the size of existing ones.

# For going from 4 disk to 5 disks, in Raid 5
btrfs balance start -v -dstripes=1..4 mnt/

If you are not using a striped profile now, but intend to convert to a striped profile in the future, always perform a full data balance after adding drives or replacing existing drives with larger ones. The stock btrfs balance tool cannot cope with special cases on filesystems with striped raid profiles, and will paint itself into a corner that will require custom userspace balancing tools to recover if you try.

To watch one can use the following:

watch "btrfs filesystem usage -T mnt/; btrfs balance status mnt/"

Convert RAID1 after mkfs with defaults

If you forgot to set the block group profile when creating the volume, run the following command:

btrfs balance start -v convert=raid1,soft mnt/

This will convert all remaining profiles that are not yet raid1.

Convert data to RAID10 with RAID1C4 for metadata

If you a have multi device setup, or you’d like to have different profiles on a single disk, e.g. RAID10 for data and RAID1C4 for metadata and system:

btrfs balance start -v -mconvert=raid1C4,soft -dconvert=raid10,soft mnt/

Compact under used chunks

If the data chunks are not balanced and used only partially, the usage filter can be used to make them more compact:

btrfs balance start -v -dusage=10 mnt/

If the percent starts from a small number, like 5 or 10, the chunks will be processed relatively quickly and will make more space available. Increasing the percentage can then make more chunks compact by relocating the data.

Chunks utilized up to 50% can be relocated to other chunks while still freeing the space. With utilization higher than 50% the chunks will be basically only moved on the devices. The actual chunk layout may help to coalesce the free space but this is a secondary effect.

for USAGE in {10..50..10} do
    btrfs balance start -v -dusage=$USAGE mnt/
done

Fix incomplete balance

If the balance is interrupted (due to reboot or cancelled) during conversion to RAID1. The following command will skip all RAID1 chunks that have been already converted and continue with what’s left to convert. Note that an interrupted conversion may leave the last chunk under utilized.

btrfs balance start convert=raid1,soft mnt/