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

6       btrfs-device - manage devices of btrfs filesystems
7

SYNOPSIS

9       btrfs device <subcommand> <args>
10

DESCRIPTION

12       The btrfs device command group is used to manage devices of the btrfs
13       filesystems.
14

DEVICE MANAGEMENT

16       Btrfs filesystem can be created on top of single or multiple block
17       devices. Data and metadata are organized in allocation profiles with
18       various redundancy policies. There’s some similarity with traditional
19       RAID levels, but this could be confusing to users familiar with the
20       traditional meaning. Due to the similarity, the RAID terminology is
21       widely used in the documentation. See mkfs.btrfs(8) for more details
22       and the exact profile capabilities and constraints.
23
24       The device management works on a mounted filesystem. Devices can be
25       added, removed or replaced, by commands provided by btrfs device and
26       btrfs replace.
27
28       The profiles can be also changed, provided there’s enough workspace to
29       do the conversion, using the btrfs balance command and namely the
30       filter convert.
31
32       Profile
33           A profile describes an allocation policy based on the
34           redundancy/replication constraints in connection with the number of
35           devices. The profile applies to data and metadata block groups
36           separately.
37
38       RAID level
39           Where applicable, the level refers to a profile that matches
40           constraints of the standard RAID levels. At the moment the
41           supported ones are: RAID0, RAID1, RAID10, RAID5 and RAID6.
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43       See the section TYPICAL USECASES for some examples.
44

SUBCOMMAND

46       add [-Kf] <device> [<device>...] <path>
47           Add device(s) to the filesystem identified by <path>.
48
49           If applicable, a whole device discard (TRIM) operation is performed
50           prior to adding the device. A device with existing filesystem
51           detected by blkid(8) will prevent device addition and has to be
52           forced. Alternatively the filesystem can be wiped from the device
53           using eg. the wipefs(8) tool.
54
55           The operation is instant and does not affect existing data. The
56           operation merely adds the device to the filesystem structures and
57           creates some block groups headers.
58
59           Options
60
61           -K|--nodiscard
62               do not perform discard (TRIM) by default
63
64           -f|--force
65               force overwrite of existing filesystem on the given disk(s)
66
67       remove <device>|<devid> [<device>|<devid>...] <path>
68           Remove device(s) from a filesystem identified by <path>
69
70           Device removal must satisfy the profile constraints, otherwise the
71           command fails. The filesystem must be converted to profile(s) that
72           would allow the removal. This can typically happen when going down
73           from 2 devices to 1 and using the RAID1 profile. See the TYPICAL
74           USECASES section below.
75
76           The operation can take long as it needs to move all data from the
77           device.
78
79           It is possible to delete the device that was used to mount the
80           filesystem. The device entry in the mount table will be replaced by
81           another device name with the lowest device id.
82
83           If the filesystem is mounted in degraded mode (-o degraded),
84           special term missing can be used for device. In that case, the
85           first device that is described by the filesystem metadata, but not
86           present at the mount time will be removed.
87
88               Note
89               In most cases, there is only one missing device in degraded
90               mode, otherwise mount fails. If there are two or more devices
91               missing (e.g. possible in RAID6), you need specify missing as
92               many times as the number of missing devices to remove all of
93               them.
94
95       delete <device>|<devid> [<device>|<devid>...] <path>
96           Alias of remove kept for backward compatibility
97
98       ready <device>
99           Wait until all devices of a multiple-device filesystem are scanned
100           and registered within the kernel module. This is to provide a way
101           for automatic filesystem mounting tools to wait before the mount
102           can start. The device scan is only one of the preconditions and the
103           mount can fail for other reasons. Normal users usually do not need
104           this command and may safely ignore it.
105
106       scan [options] [<device> [<device>...]]
107           Scan devices for a btrfs filesystem and register them with the
108           kernel module. This allows mounting multiple-device filesystem by
109           specifying just one from the whole group.
110
111           If no devices are passed, all block devices that blkid reports to
112           contain btrfs are scanned.
113
114           The options --all-devices or -d can be used as a fallback in case
115           blkid is not available. If used, behavior is the same as if no
116           devices are passed.
117
118           The command can be run repeatedly. Devices that have been already
119           registered remain as such. Reloading the kernel module will drop
120           this information. There’s an alternative way of mounting
121           multiple-device filesystem without the need for prior scanning. See
122           the mount option device.
123
124           Options
125
126           -d|--all-devices
127               Enumerate and register all devices, use as a fallback in case
128               blkid is not available.
129
130           -u|--forget
131               Unregister a given device or all stale devices if no path is
132               given, the device must be unmounted otherwise it’s an error.
133
134       stats [options] <path>|<device>
135           Read and print the device IO error statistics for all devices of
136           the given filesystem identified by <path> or for a single <device>.
137           The filesystem must be mounted. See section DEVICE STATS for more
138           information about the reported statistics and the meaning.
139
140           Options
141
142           -z|--reset
143               Print the stats and reset the values to zero afterwards.
144
145           -c|--check
146               Check if the stats are all zeros and return 0 if it is so. Set
147               bit 6 of the return code if any of the statistics is no-zero.
148               The error values is 65 if reading stats from at least one
149               device failed, otherwise it’s 64.
150
151       usage [options] <path> [<path>...]
152           Show detailed information about internal allocations in devices.
153
154           Options
155
156           -b|--raw
157               raw numbers in bytes, without the B suffix
158
159           -h|--human-readable
160               print human friendly numbers, base 1024, this is the default
161
162           -H
163               print human friendly numbers, base 1000
164
165           --iec
166               select the 1024 base for the following options, according to
167               the IEC standard
168
169           --si
170               select the 1000 base for the following options, according to
171               the SI standard
172
173           -k|--kbytes
174               show sizes in KiB, or kB with --si
175
176           -m|--mbytes
177               show sizes in MiB, or MB with --si
178
179           -g|--gbytes
180               show sizes in GiB, or GB with --si
181
182           -t|--tbytes
183               show sizes in TiB, or TB with --si
184
185       If conflicting options are passed, the last one takes precedence.
186

TYPICAL USECASES

188   STARTING WITH A SINGLE-DEVICE FILESYSTEM
189       Assume we’ve created a filesystem on a block device /dev/sda with
190       profile single/single (data/metadata), the device size is 50GiB and
191       we’ve used the whole device for the filesystem. The mount point is
192       /mnt.
193
194       The amount of data stored is 16GiB, metadata have allocated 2GiB.
195
196       ADD NEW DEVICE
197           We want to increase the total size of the filesystem and keep the
198           profiles. The size of the new device /dev/sdb is 100GiB.
199
200               $ btrfs device add /dev/sdb /mnt
201
202           The amount of free data space increases by less than 100GiB, some
203           space is allocated for metadata.
204
205       CONVERT TO RAID1
206           Now we want to increase the redundancy level of both data and
207           metadata, but we’ll do that in steps. Note, that the device sizes
208           are not equal and we’ll use that to show the capabilities of split
209           data/metadata and independent profiles.
210
211           The constraint for RAID1 gives us at most 50GiB of usable space and
212           exactly 2 copies will be stored on the devices.
213
214           First we’ll convert the metadata. As the metadata occupy less than
215           50GiB and there’s enough workspace for the conversion process, we
216           can do:
217
218               $ btrfs balance start -mconvert=raid1 /mnt
219
220           This operation can take a while, because all metadata have to be
221           moved and all block pointers updated. Depending on the physical
222           locations of the old and new blocks, the disk seeking is the key
223           factor affecting performance.
224
225           You’ll note that the system block group has been also converted to
226           RAID1, this normally happens as the system block group also holds
227           metadata (the physical to logical mappings).
228
229           What changed:
230
231           ·   available data space decreased by 3GiB, usable roughly (50 - 3)
232               + (100 - 3) = 144 GiB
233
234           ·   metadata redundancy increased
235
236           IOW, the unequal device sizes allow for combined space for data yet
237           improved redundancy for metadata. If we decide to increase
238           redundancy of data as well, we’re going to lose 50GiB of the second
239           device for obvious reasons.
240
241               $ btrfs balance start -dconvert=raid1 /mnt
242
243           The balance process needs some workspace (ie. a free device space
244           without any data or metadata block groups) so the command could
245           fail if there’s too much data or the block groups occupy the whole
246           first device.
247
248           The device size of /dev/sdb as seen by the filesystem remains
249           unchanged, but the logical space from 50-100GiB will be unused.
250
251       REMOVE DEVICE
252           Device removal must satisfy the profile constraints, otherwise the
253           command fails. For example:
254
255               $ btrfs device remove /dev/sda /mnt
256               ERROR: error removing device '/dev/sda': unable to go below two devices on raid1
257
258           In order to remove a device, you need to convert the profile in
259           this case:
260
261               $ btrfs balance start -mconvert=dup -dconvert=single /mnt
262               $ btrfs device remove /dev/sda /mnt
263

DEVICE STATS

265       The device stats keep persistent record of several error classes
266       related to doing IO. The current values are printed at mount time and
267       updated during filesystem lifetime or from a scrub run.
268
269           $ btrfs device stats /dev/sda3
270           [/dev/sda3].write_io_errs   0
271           [/dev/sda3].read_io_errs    0
272           [/dev/sda3].flush_io_errs   0
273           [/dev/sda3].corruption_errs 0
274           [/dev/sda3].generation_errs 0
275
276       write_io_errs
277           Failed writes to the block devices, means that the layers beneath
278           the filesystem were not able to satisfy the write request.
279
280       read_io_errors
281           Read request analogy to write_io_errs.
282
283       flush_io_errs
284           Number of failed writes with the FLUSH flag set. The flushing is a
285           method of forcing a particular order between write requests and is
286           crucial for implementing crash consistency. In case of btrfs, all
287           the metadata blocks must be permanently stored on the block device
288           before the superblock is written.
289
290       corruption_errs
291           A block checksum mismatched or a corrupted metadata header was
292           found.
293
294       generation_errs
295           The block generation does not match the expected value (eg. stored
296           in the parent node).
297

EXIT STATUS

299       btrfs device returns a zero exit status if it succeeds. Non zero is
300       returned in case of failure.
301
302       If the -s option is used, btrfs device stats will add 64 to the exit
303       status if any of the error counters is non-zero.
304

AVAILABILITY

306       btrfs is part of btrfs-progs. Please refer to the btrfs wiki
307       http://btrfs.wiki.kernel.org for further details.
308

SEE ALSO

310       mkfs.btrfs(8), btrfs-replace(8), btrfs-balance(8)
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314Btrfs v5.6                        04/05/2020                   BTRFS-DEVICE(8)
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