1BTRFS-BALANCE(8)                 Btrfs Manual                 BTRFS-BALANCE(8)
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NAME

6       btrfs-balance - balance block groups on a btrfs filesystem
7

SYNOPSIS

9       btrfs balance <subcommand> <args>
10

DESCRIPTION

12       The primary purpose of the balance feature is to spread block groups
13       across all devices so they match constraints defined by the respective
14       profiles. See mkfs.btrfs(8) section PROFILES for more details. The
15       scope of the balancing process can be further tuned by use of filters
16       that can select the block groups to process. Balance works only on a
17       mounted filesystem.
18
19       The balance operation is cancellable by the user. The on-disk state of
20       the filesystem is always consistent so an unexpected interruption (eg.
21       system crash, reboot) does not corrupt the filesystem. The progress of
22       the balance operation is temporarily stored as an internal state and
23       will be resumed upon mount, unless the mount option skip_balance is
24       specified.
25
26           Warning
27           running balance without filters will take a lot of time as it
28           basically rewrites the entire filesystem and needs to update all
29           block pointers.
30
31       The filters can be used to perform following actions:
32
33       ·   convert block group profiles (filter convert)
34
35       ·   make block group usage more compact (filter usage)
36
37       ·   perform actions only on a given device (filters devid, drange)
38
39       The filters can be applied to a combination of block group types (data,
40       metadata, system). Note that changing system needs the force option.
41
42           Note
43           the balance operation needs enough work space, ie. space that is
44           completely unused in the filesystem, otherwise this may lead to
45           ENOSPC reports. See the section ENOSPC for more details.
46

COMPATIBILITY

48           Note
49           The balance subcommand also exists under the btrfs filesystem
50           namespace. This still works for backward compatibility but is
51           deprecated and should not be used any more.
52
53           Note
54           A short syntax btrfs balance <path> works due to backward
55           compatibility but is deprecated and should not be used any more.
56           Use btrfs balance start command instead.
57

PERFORMANCE IMPLICATIONS

59       Balancing operations are very IO intensive and can also be quite CPU
60       intensive, impacting other ongoing filesystem operations. Typically
61       large amounts of data are copied from one location to another, with
62       corresponding metadata updates.
63
64       Depending upon the block group layout, it can also be seek heavy.
65       Performance on rotational devices is noticeably worse compared to SSDs
66       or fast arrays.
67

SUBCOMMAND

69       cancel <path>
70           cancels a running or paused balance, the command will block and
71           wait until the current blockgroup being processed completes
72
73       pause <path>
74           pause running balance operation, this will store the state of the
75           balance progress and used filters to the filesystem
76
77       resume <path>
78           resume interrupted balance, the balance status must be stored on
79           the filesystem from previous run, eg. after it was forcibly
80           interrupted and mounted again with skip_balance
81
82       start [options] <path>
83           start the balance operation according to the specified filters, no
84           filters will rewrite the entire filesystem. The process runs in the
85           foreground.
86
87               Note
88               the balance command without filters will basically rewrite
89               everything in the filesystem. The run time is potentially very
90               long, depending on the filesystem size. To prevent starting a
91               full balance by accident, the user is warned and has a few
92               seconds to cancel the operation before it starts. The warning
93               and delay can be skipped with --full-balance option.
94           Please note that the filters must be written together with the -d,
95           -m and -s options, because they’re optional and bare -d etc also
96           work and mean no filters.
97
98           Options
99
100           -d[<filters>]
101               act on data block groups, see FILTERS section for details about
102               filters
103
104           -m[<filters>]
105               act on metadata chunks, see FILTERS section for details about
106               filters
107
108           -s[<filters>]
109               act on system chunks (requires -f), see FILTERS section for
110               details about filters.
111
112           -v
113               be verbose and print balance filter arguments
114
115           -f
116               force a reduction of metadata integrity, eg. when going from
117               raid1 to single
118
119           --background|--bg
120               run the balance operation asynchronously in the background,
121               uses fork(2) to start the process that calls the kernel ioctl
122
123       status [-v] <path>
124           Show status of running or paused balance.
125
126           If -v option is given, output will be verbose.
127

FILTERS

129       From kernel 3.3 onwards, btrfs balance can limit its action to a subset
130       of the whole filesystem, and can be used to change the replication
131       configuration (e.g. moving data from single to RAID1). This
132       functionality is accessed through the -d, -m or -s options to btrfs
133       balance start, which filter on data, metadata and system blocks
134       respectively.
135
136       A filter has the following structure: type[=params][,type=...]
137
138       The available types are:
139
140       profiles=<profiles>
141           Balances only block groups with the given profiles. Parameters are
142           a list of profile names separated by "|" (pipe).
143
144       usage=<percent>, usage=<range>
145           Balances only block groups with usage under the given percentage.
146           The value of 0 is allowed and will clean up completely unused block
147           groups, this should not require any new work space allocated. You
148           may want to use usage=0 in case balance is returning ENOSPC and
149           your filesystem is not too full.
150
151           The argument may be a single value or a range. The single value N
152           means at most N percent used, equivalent to ..N range syntax.
153           Kernels prior to 4.4 accept only the single value format. The
154           minimum range boundary is inclusive, maximum is exclusive.
155
156       devid=<id>
157           Balances only block groups which have at least one chunk on the
158           given device. To list devices with ids use btrfs filesystem show.
159
160       drange=<range>
161           Balance only block groups which overlap with the given byte range
162           on any device. Use in conjunction with devid to filter on a
163           specific device. The parameter is a range specified as start..end.
164
165       vrange=<range>
166           Balance only block groups which overlap with the given byte range
167           in the filesystem’s internal virtual address space. This is the
168           address space that most reports from btrfs in the kernel log use.
169           The parameter is a range specified as start..end.
170
171       convert=<profile>
172           Convert each selected block group to the given profile name
173           identified by parameters.
174
175               Note
176               starting with kernel 4.5, the data chunks can be converted
177               to/from the DUP profile on a single device.
178
179               Note
180               starting with kernel 4.6, all profiles can be converted to/from
181               DUP on multi-device filesystems.
182
183       limit=<number>, limit=<range>
184           Process only given number of chunks, after all filters are applied.
185           This can be used to specifically target a chunk in connection with
186           other filters (drange, vrange) or just simply limit the amount of
187           work done by a single balance run.
188
189           The argument may be a single value or a range. The single value N
190           means at most N chunks, equivalent to ..N range syntax. Kernels
191           prior to 4.4 accept only the single value format. The range minimum
192           and maximum are inclusive.
193
194       stripes=<range>
195           Balance only block groups which have the given number of stripes.
196           The parameter is a range specified as start..end. Makes sense for
197           block group profiles that utilize striping, ie. RAID0/10/5/6. The
198           range minimum and maximum are inclusive.
199
200       soft
201           Takes no parameters. Only has meaning when converting between
202           profiles. When doing convert from one profile to another and soft
203           mode is on, chunks that already have the target profile are left
204           untouched. This is useful e.g. when half of the filesystem was
205           converted earlier but got cancelled.
206
207           The soft mode switch is (like every other filter) per-type. For
208           example, this means that we can convert metadata chunks the "hard"
209           way while converting data chunks selectively with soft switch.
210
211       Profile names, used in profiles and convert are one of: raid0, raid1,
212       raid10, raid5, raid6, dup, single. The mixed data/metadata profiles can
213       be converted in the same way, but it’s conversion between mixed and
214       non-mixed is not implemented. For the constraints of the profiles
215       please refer to mkfs.btrfs(8), section PROFILES.
216

ENOSPC

218       The way balance operates, it usually needs to temporarily create a new
219       block group and move the old data there, before the old block group can
220       be removed. For that it needs the work space, otherwise it fails for
221       ENOSPC reasons. This is not the same ENOSPC as if the free space is
222       exhausted. This refers to the space on the level of block groups, which
223       are bigger parts of the filesystem that contain many file extents.
224
225       The free work space can be calculated from the output of the btrfs
226       filesystem show command:
227
228              Label: 'BTRFS'  uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
229                      Total devices 2 FS bytes used 77.03GiB
230                      devid    1 size 53.90GiB used 51.90GiB path /dev/sdc2
231                      devid    2 size 53.90GiB used 51.90GiB path /dev/sde1
232
233       size - used = free work space 53.90GiB - 51.90GiB = 2.00GiB
234
235       An example of a filter that does not require workspace is usage=0. This
236       will scan through all unused block groups of a given type and will
237       reclaim the space. After that it might be possible to run other
238       filters.
239
240       CONVERSIONS ON MULTIPLE DEVICES
241
242       Conversion to profiles based on striping (RAID0, RAID5/6) require the
243       work space on each device. An interrupted balance may leave partially
244       filled block groups that consume the work space.
245

EXAMPLES

247       A more comprehensive example when going from one to multiple devices,
248       and back, can be found in section TYPICAL USECASES of btrfs-device(8).
249
250   MAKING BLOCK GROUP LAYOUT MORE COMPACT
251       The layout of block groups is not normally visible; most tools report
252       only summarized numbers of free or used space, but there are still some
253       hints provided.
254
255       Let’s use the following real life example and start with the output:
256
257           $ btrfs filesystem df /path
258           Data, single: total=75.81GiB, used=64.44GiB
259           System, RAID1: total=32.00MiB, used=20.00KiB
260           Metadata, RAID1: total=15.87GiB, used=8.84GiB
261           GlobalReserve, single: total=512.00MiB, used=0.00B
262
263       Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is
264       about 85%. How can we can interpret that:
265
266       ·   chunks are filled by 85% on average, ie. the usage filter with
267           anything smaller than 85 will likely not affect anything
268
269       ·   in a more realistic scenario, the space is distributed unevenly, we
270           can assume there are completely used chunks and the remaining are
271           partially filled
272
273       Compacting the layout could be used on both. In the former case it
274       would spread data of a given chunk to the others and removing it. Here
275       we can estimate that roughly 850 MiB of data have to be moved (85% of a
276       1 GiB chunk).
277
278       In the latter case, targeting the partially used chunks will have to
279       move less data and thus will be faster. A typical filter command would
280       look like:
281
282           # btrfs balance start -dusage=50 /path
283           Done, had to relocate 2 out of 97 chunks
284
285           $ btrfs filesystem df /path
286           Data, single: total=74.03GiB, used=64.43GiB
287           System, RAID1: total=32.00MiB, used=20.00KiB
288           Metadata, RAID1: total=15.87GiB, used=8.84GiB
289           GlobalReserve, single: total=512.00MiB, used=0.00B
290
291       As you can see, the total amount of data is decreased by just 1 GiB,
292       which is an expected result. Let’s see what will happen when we
293       increase the estimated usage filter.
294
295           # btrfs balance start -dusage=85 /path
296           Done, had to relocate 13 out of 95 chunks
297
298           $ btrfs filesystem df /path
299           Data, single: total=68.03GiB, used=64.43GiB
300           System, RAID1: total=32.00MiB, used=20.00KiB
301           Metadata, RAID1: total=15.87GiB, used=8.85GiB
302           GlobalReserve, single: total=512.00MiB, used=0.00B
303
304       Now the used/total ratio is about 94% and we moved about 74G - 68G = 6G
305       of data to the remaining blockgroups, ie. the 6GiB are now free of
306       filesystem structures, and can be reused for new data or metadata block
307       groups.
308
309       We can do a similar exercise with the metadata block groups, but this
310       should not typically be necessary, unless the used/total ratio is
311       really off. Here the ratio is roughly 50% but the difference as an
312       absolute number is "a few gigabytes", which can be considered normal
313       for a workload with snapshots or reflinks updated frequently.
314
315           # btrfs balance start -musage=50 /path
316           Done, had to relocate 4 out of 89 chunks
317
318           $ btrfs filesystem df /path
319           Data, single: total=68.03GiB, used=64.43GiB
320           System, RAID1: total=32.00MiB, used=20.00KiB
321           Metadata, RAID1: total=14.87GiB, used=8.85GiB
322           GlobalReserve, single: total=512.00MiB, used=0.00B
323
324       Just 1 GiB decrease, which possibly means there are block groups with
325       good utilization. Making the metadata layout more compact would in turn
326       require updating more metadata structures, ie. lots of IO. As running
327       out of metadata space is a more severe problem, it’s not necessary to
328       keep the utilization ratio too high. For the purpose of this example,
329       let’s see the effects of further compaction:
330
331           # btrfs balance start -musage=70 /path
332           Done, had to relocate 13 out of 88 chunks
333
334           $ btrfs filesystem df .
335           Data, single: total=68.03GiB, used=64.43GiB
336           System, RAID1: total=32.00MiB, used=20.00KiB
337           Metadata, RAID1: total=11.97GiB, used=8.83GiB
338           GlobalReserve, single: total=512.00MiB, used=0.00B
339
340   GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
341       Normally the balance operation needs a work space, to temporarily move
342       the data before the old block groups gets removed. If there’s no work
343       space, it ends with no space left.
344
345       There’s a special case when the block groups are completely unused,
346       possibly left after removing lots of files or deleting snapshots.
347       Removing empty block groups is automatic since 3.18. The same can be
348       achieved manually with a notable exception that this operation does not
349       require the work space. Thus it can be used to reclaim unused block
350       groups to make it available.
351
352           # btrfs balance start -dusage=0 /path
353
354       This should lead to decrease in the total numbers in the btrfs
355       filesystem df output.
356

EXIT STATUS

358       btrfs balance returns a zero exit status if it succeeds. Non zero is
359       returned in case of failure.
360

AVAILABILITY

362       btrfs is part of btrfs-progs. Please refer to the btrfs wiki
363       http://btrfs.wiki.kernel.org for further details.
364

SEE ALSO

366       mkfs.btrfs(8), btrfs-device(8)
367
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369
370Btrfs v5.1                        05/17/2019                  BTRFS-BALANCE(8)
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