+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, ie. 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 blockgroups, ie. 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, ie. 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.
projects / platform / upstream / btrfs-progs.git / blobdiff