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[PATCH] md: document sysfs usage of md, and make a couple of small re…


Document in Documentation/md.txt the files that now appear in sysfs, and make
a couple of small refinements to exactly when 'level' and 'raid_disks' are
empty, to make it match the documentation.

Signed-off-by: Neil Brown <>
Acked-by: Greg Kroah-Hartman <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
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1 parent 7eec314 commit bb636547b02411ca5eef87b1d030ea3fc090a717 @neilbrown neilbrown committed with Linus Torvalds Nov 9, 2005
Showing with 122 additions and 1 deletion.
  1. +119 −0 Documentation/md.txt
  2. +3 −1 drivers/md/md.c
119 Documentation/md.txt
@@ -116,3 +116,122 @@ and it's role in the array.
Once started with RUN_ARRAY, uninitialized spares can be added with
+MD devices in sysfs
+md devices appear in sysfs (/sys) as regular block devices,
+ /sys/block/md0
+Each 'md' device will contain a subdirectory called 'md' which
+contains further md-specific information about the device.
+All md devices contain:
+ level
+ a text file indicating the 'raid level'. This may be a standard
+ numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
+ other name such as "linear" or "multipath".
+ If no raid level has been set yet (array is still being
+ assembled), this file will be empty.
+ raid_disks
+ a text file with a simple number indicating the number of devices
+ in a fully functional array. If this is not yet known, the file
+ will be empty. If an array is being resized (not currently
+ possible) this will contain the larger of the old and new sizes.
+As component devices are added to an md array, they appear in the 'md'
+directory as new directories named
+ dev-XXX
+where XXX is a name that the kernel knows for the device, e.g. hdb1.
+Each directory contains:
+ block
+ a symlink to the block device in /sys/block, e.g.
+ /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
+ super
+ A file containing an image of the superblock read from, or
+ written to, that device.
+ state
+ A file recording the current state of the device in the array
+ which can be a comma separated list of
+ faulty - device has been kicked from active use due to
+ a detected fault
+ in_sync - device is a fully in-sync member of the array
+ spare - device is working, but not a full member.
+ This includes spares that are in the process
+ of being recoverred to
+ This list make grow in future.
+An active md device will also contain and entry for each active device
+in the array. These are named
+ rdNN
+where 'NN' is the possition in the array, starting from 0.
+So for a 3 drive array there will be rd0, rd1, rd2.
+These are symbolic links to the appropriate 'dev-XXX' entry.
+Thus, for example,
+ cat /sys/block/md*/md/rd*/state
+will show 'in_sync' on every line.
+Active md devices for levels that support data redundancy (1,4,5,6)
+also have
+ sync_action
+ a text file that can be used to monitor and control the rebuild
+ process. It contains one word which can be one of:
+ resync - redundancy is being recalculated after unclean
+ shutdown or creation
+ recover - a hot spare is being built to replace a
+ failed/missing device
+ idle - nothing is happening
+ check - A full check of redundancy was requested and is
+ happening. This reads all block and checks
+ them. A repair may also happen for some raid
+ levels.
+ repair - A full check and repair is happening. This is
+ similar to 'resync', but was requested by the
+ user, and the write-intent bitmap is NOT used to
+ optimise the process.
+ This file is writable, and each of the strings that could be
+ read are meaningful for writing.
+ 'idle' will stop an active resync/recovery etc. There is no
+ guarantee that another resync/recovery may not be automatically
+ started again, though some event will be needed to trigger
+ this.
+ 'resync' or 'recovery' can be used to restart the
+ corresponding operation if it was stopped with 'idle'.
+ 'check' and 'repair' will start the appropriate process
+ providing the current state is 'idle'.
+ mismatch_count
+ When performing 'check' and 'repair', and possibly when
+ performing 'resync', md will count the number of errors that are
+ found. The count in 'mismatch_cnt' is the number of sectors
+ that were re-written, or (for 'check') would have been
+ re-written. As most raid levels work in units of pages rather
+ than sectors, this my be larger than the number of actual errors
+ by a factor of the number of sectors in a page.
+Each active md device may also have attributes specific to the
+personality module that manages it.
+These are specific to the implementation of the module and could
+change substantially if the implementation changes.
+These currently include
+ stripe_cache_size (currently raid5 only)
+ number of entries in the stripe cache. This is writable, but
+ there are upper and lower limits (32768, 16). Default is 128.
+ strip_cache_active (currently raid5 only)
+ number of active entries in the stripe cache
4 drivers/md/md.c
@@ -1727,7 +1727,7 @@ static ssize_t
level_show(mddev_t *mddev, char *page)
mdk_personality_t *p = mddev->pers;
- if (p == NULL)
+ if (p == NULL && mddev->raid_disks == 0)
return 0;
if (mddev->level >= 0)
return sprintf(page, "RAID-%d\n", mddev->level);
@@ -1740,6 +1740,8 @@ static struct md_sysfs_entry md_level = __ATTR_RO(level);
static ssize_t
raid_disks_show(mddev_t *mddev, char *page)
+ if (mddev->raid_disks == 0)
+ return 0;
return sprintf(page, "%d\n", mddev->raid_disks);

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