c8h4/btrfs-progs
1
0
Fork 0
Code Issues Pull requests Actions Packages Projects Releases Wiki Activity

btrfs-progs: docs, enhance the mount option manual page

Browse source 
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2016-01-08 19:33:02 +01:00
parent 154c389e92
commit cf0fce7ed4
1 changed files with 197 additions and 95 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

292
Documentation/btrfs-mount.asciidoc
View file

@ -3,30 +3,42 @@ btrfs-mount(5)
NAME
----
btrfs-mount - mount options and supported file attributes for the btrfs filesystem
btrfs-mount - topics about the BTRFS filesystem (mount options, supported file attributes and other)
DESCRIPTION
-----------
This document describes mount options specific to the btrfs filesystem.
Other generic mount options are available,and are described in the
`mount`(8) manpage.
This document describes topics related to BTRFS that are not specific to the
tools.
MOUNT OPTIONS
-------------
This section describes mount options specific to BTRFS. For the generic mount
options please refer to `mount`(8) manpage.
*alloc_start='bytes'*::
(default: 1M, minimum: 1M)
+
Debugging option to force all block allocations above a certain
byte threshold on each block device. The value is specified in
bytes, optionally with a K, M, or G suffix, case insensitive.
Default is 1MB.
bytes, optionally with a K, M, or G suffix (case insensitive).
+
This option was used for testing and has not practial use, it's slated to be
removed in the future.
*autodefrag*::
*noautodefrag*::
(since: 3.0, default: off) +
Disable/enable auto defragmentation.
Auto defragmentation detects small random writes into files and queue
them up for the defrag process. Works best for small files;
(since: 3.0, default: off)
+
Enable automatic file defragmentation.
When enabled, small random writes into files (in a range of tens of kilobytes,
currently it's 64K) are detected and queued up for the defragmentation process.
Not well suited for large database workloads.
+
The read latency may increase due to reading the adjacent blocks that make up the
range for defragmentation, successive write will merge the blocks in the new
location.
+
WARNING: Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as
well as with Linux stable kernel versions ≥ 3.10.31, ≥ 3.12.12 or
≥ 3.13.4 will break up the ref-links of CoW data (for example files
@ -37,7 +49,8 @@ broken up ref-links.
*check_int*::
*check_int_data*::
*check_int_print_mask='value'*::
(since: 3.0, default: off) +
(since: 3.0, default: off)
+
These debugging options control the behavior of the integrity checking
module (the BTRFS_FS_CHECK_INTEGRITY config option required). +
+
@ -56,7 +69,8 @@ See comments at the top of 'fs/btrfs/check-integrity.c'
for more info.
*commit='seconds'*::
(since: 3.12, default: 30) +
(since: 3.12, default: 30)
+
Set the interval of periodic commit. Higher
values defer data being synced to permanent storage with obvious
consequences when the system crashes. The upper bound is not forced,
@ -66,7 +80,8 @@ but a warning is printed if it's more than 300 seconds (5 minutes).
*compress='type'*::
*compress-force*::
*compress-force='type'*::
(default: off) +
(default: off)
+
Control BTRFS file data compression. Type may be specified as 'zlib',
'lzo' or 'no' (for no compression, used for remounting). If no type
is specified, 'zlib' is used. If compress-force is specified,
@ -75,37 +90,51 @@ all files will be compressed, whether or not they compress well.
NOTE: If compression is enabled, 'nodatacow' and 'nodatasum' are disabled.
*degraded*::
(default: off) +
Allow mounts to continue with missing devices. A read-write mount may
fail with too many devices missing, for example if a stripe member
is completely missing.
(default: off)
+
Allow mounts with less devices than the raid profile constraints
require. A read-write mount (or remount) may fail with too many devices
missing, for example if a stripe member is completely missing from RAID0.
*device='devicepath'*::
Specify a device during mount so that ioctls on the control device
can be avoided. Especially useful when trying to mount a multi-device
setup as root. May be specified multiple times for multiple devices.
Specify a path to a device that will be scanned for BTRFS filesystem during
mount. This is usually done automatically by a device manager (like udev) or
using the *btrfs device scan* command (eg. run from the initial ramdisk). In
cases where this is not possible the 'device' mount option can help.
+
NOTE: booting eg. a RAID1 system may fail even if all filesystem's 'device'
paths are provided as the actual device nodes may not be discovered by the
system at that point.
*discard*::
*nodiscard*::
(default: off) +
Disable/enable discard mount option.
Discard issues frequent commands to let the block device reclaim space
freed by the filesystem.
This is useful for SSD devices, thinly provisioned
LUNs and virtual machine images, but may have a significant
performance impact. (The fstrim command is also available to
initiate batch trims from userspace).
(default: off)
+
Enable discarding of freed file blocks using TRIM operation. This is useful
for SSD devices, thinly provisioned LUNs or virtual machine images where the
backing device understands the operation. Depending on support of the
underlying device, the operation may severly hurt performance in case the TRIM
operation is synchronous (eg. with SATA devices up to revision 3.0).
+
If discarding is not necessary to be done at the block freeing time, there's
*fstrim* tool that lets the filesystem discard all free blocks in a batch,
possibly not much interfering with other operations.
*enospc_debug*::
(default: off) +
Disable/enable debugging option to be more verbose in some ENOSPC conditions.
(default: off)
+
Enable verbose output for some ENOSPC conditions. It's safe to use but can
be noisy if the system hits reaches near-full state.
*fatal_errors='action'*::
(since: 3.4, default: bug) +
Action to take when encountering a fatal error. +
(since: 3.4, default: bug)
+
Action to take when encountering a fatal error.
+
*bug*::::
'BUG()' on a fatal error, the system will stay in the crashed state and may be
still partially usable, but reboot is required for full operation +
still partially usable, but reboot is required for full operation
+
*panic*::::
'panic()' on a fatal error, depending on other system configuration, this may
be followed by a reboot. Please refer to the documentation of kernel boot
@ -113,82 +142,144 @@ parameters, eg. 'panic', 'oops' or 'crashkernel'.
*flushoncommit*::
*noflushoncommit*::
(default: on) +
The `flushoncommit` mount option forces any data dirtied by a write in a
prior transaction to commit as part of the current commit. This makes
the committed state a fully consistent view of the file system from the
application's perspective (i.e., it includes all completed file system
operations). This was previously the behavior only when a snapshot is
created.
(default: on)
+
This option forces any data dirtied by a write in a prior transaction to commit
as part of the current commit. This makes the committed state a fully
consistent view of the file system from the application's perspective (i.e., it
includes all completed file system operations). This was previously the
behavior only when a snapshot was created.
+
Disabling flushing may improve performance but is not crash-safe.
*inode_cache*::
*noinode_cache*::
(since: 3.0, default: off) +
Enable free inode number caching. Defaults to off due to an overflow
problem when the free space crcs don't fit inside a single page.
(since: 3.0, default: off)
+
Enable free inode number caching. Not recommended to use unless files on your
filesystem get assigned inode numbers that are approaching 2^64^. Normally, new
files in each subvolume get assigned incrementally (plus one from the last
time) and are not reused. The mount option turns on caching of the existing
inode numbers and reuse of inode numbers of deleted files.
+
This option may slow down your system at first run, or after mounting without
the option.
+
NOTE: Defaults to off due to a potential overflow problem when the free space
checksums don't fit inside a single page.
*max_inline='bytes'*::
(default: min(8192, page size) )
+
Specify the maximum amount of space, in bytes, that can be inlined in
a metadata B-tree leaf. The value is specified in bytes, optionally
with a K, M, or G suffix, case insensitive. In practice, this value
is limited by the root sector size, with some space unavailable due
to leaf headers. For a 4k sectorsize, max inline data is ~3900 bytes.
with a K suffix (case insensitive). In practice, this value
is limited by the filesystem block size (named 'sectorsize' at mkfs time),
and memory page size of the system. In case of sectorsize limit, there's
some space unavailable due to leaf headers. For example, a 4k sectorsize, max
inline data is ~3900 bytes.
+
Inlining can be completely turned off specifying 0. This will increase data
block slack if file sizes are much smaller than block size but will reduce
metadata consumption in return.
*metadata_ratio='value'*::
Specify that 1 metadata chunk should be allocated after every
'value' data chunks. Off by default.
(default: 0, internal logic)
+
Specifies that 1 metadata chunk should be allocated after every 'value' data
chunks. Default behaviour depends on internal logic, some percent of unused
metadata space is attempted to be maintained but is not always possible if
there's not space left for chunk allocation. The option could be useful to
override the internal logic in favor of the metadata allocation if the expected
workload is supposed to be metadata intense (snapshots, reflinks, xattrs,
inlined files).
*acl*::
*noacl*::
(default: on) +
(default: on)
+
Enable/disable support for Posix Access Control Lists (ACLs). See the
`acl`(5) manual page for more information about ACLs.
*barrier*::
*nobarrier*::
(default: on) +
ensure that certain IOs make it through the device cache and are on
persistent storage. If disabled on a device with a volatile
(non-battery-backed) write-back cache, nobarrier option will lead to
filesystem corruption on a system crash or power loss.
(default: on)
+
Ensure that all IO write operations make it through the device cache and are stored
permanently when the filesystem is at it's consistency checkpoint. This
typically means that a flush command is sent to the device that will
synchronize all pending data and ordinary metadata blocks, then writes the
superblock and issues another flush.
+
The write flushes incur a slight hit and also prevent the IO block
scheduler to reorder requests in more effective way. Disabling barriers gets
rid of that penalty but will most certainly lead to a corrupted filesystem in
case of a crash or power loss. The ordinary metadata blocks could be yet
unwrittent at the time the new superblock is stored permanently, expecting that
the block pointers to metadata were stored permanently before.
+
On a device with a volatile battery-backed write-back cache, the 'nobarrier'
option will not lead to filesystem corruption as the pending blocks are
supposed to make it to the permanent storage.
*datacow*::
*nodatacow*::
(default: on) +
Enable/disable data copy-on-write for newly created files.
Nodatacow implies nodatasum, and disables all compression.
(default: on)
+
Enable data copy-on-write for newly created files.
'Nodatacow' implies 'nodatasum', and disables 'compression'. All files created
under 'nodatacow' are also set the NOCOW file attribute (see `chattr`(1)).
*datasum*::
*nodatasum*::
(default: on) +
Enable/disable data checksumming for newly created files.
Datasum implies datacow.
(default: on)
+
Enable data checksumming for newly created files.
'Datasum' implies 'datacow', ie. the normal mode of operation. All files created
under 'nodatasum' inherit the "no checksums" property, however there's no
corresponding file attribute (see `chattr`(1)).
*treelog*::
*notreelog*::
(default: on) +
Enable/disable the tree logging used for fsync and O_SYNC writes.
(default: on)
+
Enable the tree logging used for 'fsync' and 'O_SYNC' writes. The tree log
stores changes without the need of a full filesystem sync. The log operations
are flushed at sync and transaction commit. If the system crashes between two
such syncs, the pending tree log operations are replayed during mount.
+
WARNING: currently, the tree log is replayed even with a read-only mount!
+
The tree log could contain new files/directories, these would not exist on
a mounted filesystm if the log is not replayed.
*recovery*::
(since: 3.2, default: off) +
(since: 3.2, default: off)
+
Enable autorecovery attempts if a bad tree root is found at mount time.
Currently this scans a list of several previous tree roots and tries to
use the first readable.
Currently this scans a backup list of several previous tree roots and tries to
use the first readable. This can be used with read-only mounts as well.
*rescan_uuid_tree*::
(since: 3.12, default: off) +
(since: 3.12, default: off)
+
Force check and rebuild procedure of the UUID tree. This should not
normally be needed.
*skip_balance*::
(since: 3.3, default: off) +
(since: 3.3, default: off)
+
Skip automatic resume of interrupted balance operation after mount.
May be resumed with "btrfs balance resume."
May be resumed with *btrfs balance resume* or the paused state can be removed
by *btrfs balance cancel*.
*nospace_cache*::
(since: 3.2) +
Disable freespace cache loading without clearing the cache.
(since: 3.2)
+
Disable freespace cache loading without clearing the cache and the free space
cache will not be used during the mount. This affects performance as searching
for new free blocks could take longer. On the other hand, managing the space
cache consumes some resources.
*clear_cache*::
Force clearing and rebuilding of the disk space cache if something
@ -197,38 +288,47 @@ has gone wrong.
*ssd*::
*nossd*::
*ssd_spread*::
Options to control ssd allocation schemes. By default, BTRFS will
enable or disable ssd allocation heuristics depending on whether a
rotational or nonrotational disk is in use. The ssd and nossd options
can override this autodetection. +
The ssd_spread mount option attempts to allocate into big chunks
of unused space, and may perform better on low-end ssds. ssd_spread
implies ssd, enabling all other ssd heuristics as well.
(default: SSD autodetected)
+
Options to control SSD allocation schemes. By default, BTRFS will
enable or disable SSD allocation heuristics depending on whether a
rotational or nonrotational disk is in use. The 'ssd' and 'nossd' options
can override this autodetection.
+
The 'ssd_spread' mount option attempts to allocate into bigger and aligned
chunks of unused space, and may perform better on low-end SSDs. 'ssd_spread'
implies 'ssd', enabling all other SSD heuristics as well.
*subvol='path'*::
Mount subvolume at 'path' rather than the root subvolume. The
'path' is relative to the top level subvolume.
Mount subvolume from 'path' rather than the toplevel subvolume. The
'path' is absolute (ie. starts at the toplevel subvolume).
This mount option overrides the default subvolume set for the given filesystem.
*subvolid='ID'*::
Mount subvolume specified by an ID number rather than the root subvolume.
This allows mounting of subvolumes which are not in the root of the mounted
filesystem.
You can use "btrfs subvolume list" to see subvolume ID numbers.
*subvolid='subvolid'*::
Mount subvolume specified by a 'subvolid' number rather than the toplevel
subvolume. You can use *btrfs subvolume list* to see subvolume ID numbers.
This mount option overrides the default subvolume set for the given filesystem.
*subvolrootid='objectid'*::
(deprecated) +
Mount subvolume specified by 'objectid' rather than the root subvolume.
This allows mounting of subvolumes which are not in the root of the mounted
filesystem.
You can use "btrfs subvolume show" to see the object ID for a subvolume.
(irrelevant since: 3.2, formally deprecated since: 3.10)
+
A workaround option from times (pre 3.2) when it was not possible to mount a
subvolume that did not reside directly under the toplevel subvolume.
*thread_pool='number'*::
The number of worker threads to allocate. The default number is equal
to the number of CPUs + 2, or 8, whichever is smaller.
(default: min(NRCPUS + 2, 8) )
+
The number of worker threads to allocate. NRCPUS is number of on-line CPUs
detected at the time of mount. Small number leads to less parallelism in
processing data and metadata, higher numbers could lead to a performance due to
increased locking contention, cache-line bouncing or costly data transfers
between local CPU memories.
*user_subvol_rm_allowed*::
(default: off) +
Allow subvolumes to be deleted by a non-root user. Use with caution.
(default: off)
+
Allow subvolumes to be deleted by their respective owner. Otherwise, only the
root user can do that.
FILE ATTRIBUTES
---------------
@ -258,7 +358,9 @@ For descriptions of these attribute flags, please refer to the
SEE ALSO
--------
`acl`(5),
`btrfs`(8),
`chattr`(1),
`fstrim`(8),
`mkfs.btrfs`(8),
`mount`(8),
`btrfs`(8)
`mount`(8)