btrfs-balance: add page (#7453)

pages/linux/btrfs-balance.md

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+# btrfs balance
+
+> Balance block groups on a btrfs filesystem.
+> More information: <https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balance>.
+
+- Show the status of a running or paused balance operation:
+
+`sudo btrfs balance status {{path/to/btrfs_filesystem}}`
+
+- Balance all block groups (slow; rewrites all blocks in filesystem):
+
+`sudo btrfs balance start {{path/to/btrfs_filesystem}}`
+
+- Balance data block groups which are less than 15% utilized, running the operation in the background:
+
+`sudo btrfs balance start --bg -dusage={{15}} {{path/to/btrfs_filesystem}}`
+
+- Balance a max of 10 metadata chunks with less than 20% utilization and at least 1 chunk on a given device `devid` (see `btrfs filesystem show`):
+
+`sudo btrfs balance start -musage={{20}},limit={{10}},devid={{devid}} {{path/to/btrfs_filesystem}}`
+
+- Convert data blocks to the raid6 and metadata to raid1c3 (see mkfs.btrfs(8) for profiles):
+
+`sudo btrfs balance start -dconvert={{raid6}} -mconvert={{raid1c3}} {{path/to/btrfs_filesystem}}`
+
+- Convert data blocks to raid1, skipping already converted chunks (e.g. after a previous cancelled conversion operation):
+
+`sudo btrfs balance start -dconvert={{raid1}},soft {{path/to/btrfs_filesystem}}`
+
+- Cancel, pause, or resume a running or paused balance operation:
+
+`sudo btrfs balance {{cancel|pause|resume}} {{path/to/btrfs_filesystem}}`