Balance Filters

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From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the full filesystem, and can be used to change the replication configuration (e.g. moving data from single to RAID-1). This functionality is accessed through the -d, -m or -s options to btrfs balance start, which filter on data, metadata and system blocks respectively.

A filter has the following stucture:

type=params[,type=...]

The available types are:

profiles
Balances only block groups with the given replication profiles. Parameters are a list of profile names separated by |.
usage
Balances only block groups with usage under the given percentage.
devid
Balances only block groups which have at least one chunk on the given device (by btrfs device ID -- use btrfs fi show to list device IDs)
drange
Balances 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
Balances 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
Convert each selected block group to the given profile name.
soft
Takes no parameters. Only has meaning when converting between profiles.
When doing convert from one profile to another and soft mode is on, restriper won't touch chunks that already have the target profile. This is useful if e.g. half of the FS was converted earlier.
The soft mode switch is (like every other filter) per-type. This means that we can convert for example meta chunks the "hard" way while converting data chunks selectively with soft switch.

Profile names, used in profiles and convert are one of: raid0, raid1, raid10, dup, single.

Examples

Convert data to raid1 after mkfs with defaults

You want to increase redundancy for data as well, with at least 2 devices you can do:

$ mkfs.btrfs /dev/sda1 /dev/sdb1
$ mount /dev/sda1 /mnt
[...]
$ btrfs fi df /mnt

output:

Data, RAID0: total=409.50MB, used=0.00
Data: total=8.00MB, used=0.00
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=204.75MB, used=24.00KB
Metadata: total=8.00MB, used=0.00
$ btrfs fi balance start -dconvert=raid1 /mnt
$ btrfs fi df /mnt

output:

Data, RAID1: total=819.12MB, used=128.00KB
System, RAID1: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=460.75MB, used=24.00KB
Metadata: total=8.00MB, used=0.00

Convert data and metadata to raid1 after mkfs with defaults

The above example won't give you an actual RAID1 configuration as it only mirrors. You can see this if your output after running the above commands looks like the following:

Data, RAID1: total=819.12MB, used=128.00KB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=460.75MB, used=24.00KB
Metadata: total=8.00MB, used=0.00

You can also see it didn't work out the way you might have wanted to because

$ btrfs fi show 

will report an unbalanced amount of space used on your devices.

To actually get RAID1 where you are left with a working copy of the filesystem after you lose one of your devices, you must specify the following to rebalance to RAID1:

$ btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt
$ btrfs fi df /mnt


For more examples read this email: patch

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