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-rw-r--r--Documentation/btrfs-man5.asciidoc47
1 files changed, 27 insertions, 20 deletions
diff --git a/Documentation/btrfs-man5.asciidoc b/Documentation/btrfs-man5.asciidoc
index caa9390b..acc4e429 100644
--- a/Documentation/btrfs-man5.asciidoc
+++ b/Documentation/btrfs-man5.asciidoc
@@ -319,25 +319,32 @@ May be resumed with *btrfs balance resume* or the paused state can be removed
by *btrfs balance cancel*. The default behaviour is to start interrutpd balance.
*space_cache*::
-*space_cache=v2*::
+*space_cache='version'*::
*nospace_cache*::
-('nospace_cache' since: 3.2, 'space_cache=v2' since 4.5, default: on)
-+
-Options to control the free space cache. This affects performance as searching
-for new free blocks could take longer if the space cache is not enabled. On the
-other hand, managing the space cache consumes some resources. It can be
-disabled without clearing at mount time.
-+
-There are two implementations of how the space is tracked. The safe default is
-'v1'. On large filesystems (many-terabytes) and certain workloads the 'v1'
-performance may degrade. This problem is addressed by 'v2', that is based on
-b-trees, sometimes referred to as 'free-space-tree'.
-+
-'Compatibility notes:'
-+
-* the 'v2' has to be enabled manually at mount time, once
-* kernel without 'v2' support will be able to mount the filesystem in read-only mode
-* 'v2' can be removed by mounting with 'clear_cache'
+('nospace_cache' since: 3.2, 'space_cache=v1' and 'space_cache=v2' since 4.5, default: 'space_cache=v1')
++
+Options to control the free space cache. The free space cache greatly improves
+performance when reading block group free space into memory. However, managing
+the space cache consumes some resources, including a small amount of disk
+space.
++
+There are two implementations of the free space cache. The original
+implementation, 'v1', is the safe default. The 'v1' space cache can be disabled
+at mount time with 'nospace_cache' without clearing.
++
+On very large filesystems (many terabytes) and certain workloads, the
+performance of the 'v1' space cache may degrade drastically. The 'v2'
+implementation, which adds a new B-tree called the free space tree, addresses
+this issue. Once enabled, the 'v2' space cache will always be used and cannot
+be disabled unless it is cleared. Use 'clear_cache,space_cache=v1' or
+'clear_cache,nospace_cache' to do so. If 'v2' is enabled, kernels without 'v2'
+support will only be able to mount the filesystem in read-only mode. The
+`btrfs(8)` command currently only has read-only support for 'v2'. A read-write
+command may be run on a 'v2' filesystem by clearing the cache, running the
+command, and then remounting with 'space_cache=v2'.
++
+If a version is not explicitly specified, the default implementation will be
+chosen, which is 'v1' as of 4.9.
*ssd*::
*nossd*::
@@ -434,7 +441,7 @@ dump-super device* will dump a superblock, you can map the value of
after mkfs, on a mounted filesystem::
The features of a filesystem (with a given UUID) are listed in
`/sys/fs/btrfs/UUID/features/`, one file per feature. The status of is stored
-insid the file. The value '1' is for enabled, '0' means the feature was had
+insid the file. The value '1' is for enabled, '0' means the feature had
been enabled at the mount time and turned off afterwards.
+
Whether a particular feature can be turned on a mounted filesystem can be found
@@ -562,7 +569,7 @@ crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
The device accepts some ioctl calls that can perform following actions on the
filesyste module:
-* scan devices for btrfs filesytem (ie. to let multi-device filesystems mount
+* scan devices for btrfs filesystem (ie. to let multi-device filesystems mount
automatically) and register them with the kernel module
* similar to scan, but also wait until the device scanning process is finished
for a given filesystem