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 and will be resumed upon
23       mount, unless the mount option skip_balance is specified.
24
25           Warning
26           running balance without filters will take a lot of time as it
27           basically rewrites the entire filesystem and needs to update all
28           block pointers.
29
30       The filters can be used to perform following actions:
31
32       ·   convert block group profiles (filter convert)
33
34       ·   make block group usage more compact (filter usage)
35
36       ·   perform actions only on a given device (filters devid, drange)
37
38       The filters can be applied to a combination of block group types (data,
39       metadata, system). Note that changing system needs the force option.
40
41           Note
42           the balance operation needs enough work space, ie. space that is
43           completely unused in the filesystem, otherwise this may lead to
44           ENOSPC reports. See the section ENOSPC for more details.
45

COMPATIBILITY

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

PERFORMANCE IMPLICATIONS

58       Balance operation is intense namely in the IO respect, but can be also
59       CPU intense. It affects other actions on the filesystem. There are
60       typically lots of data being copied from one location to another, and
61       lots of metadata get updated.
62
63       Depending on the actual block group layout, it can be also seek-heavy.
64       The performance on rotational devices is noticeably worse than on SSDs
65       or fast arrays.
66

SUBCOMMAND

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

FILTERS

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

ENOSPC

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

EXAMPLES

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

EXIT STATUS

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

AVAILABILITY

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

SEE ALSO

364       mkfs.btrfs(8), btrfs-device(8)
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367
368Btrfs v4.9.1                      08/06/2017                  BTRFS-BALANCE(8)
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