[PATCH] md: allow md array component size to be accessed and set via sysfs

[safe/jmp/linux-2.6] / Documentation / md.txt

Tools that manage md devices can be found at
   http://www.<country>.kernel.org/pub/linux/utils/raid/....


Boot time assembly of RAID arrays
---------------------------------

You can boot with your md device with the following kernel command
lines:

for old raid arrays without persistent superblocks:
  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn

for raid arrays with persistent superblocks
  md=<md device no.>,dev0,dev1,...,devn
or, to assemble a partitionable array:
  md=d<md device no.>,dev0,dev1,...,devn
  
md device no. = the number of the md device ... 
              0 means md0, 
              1 md1,
              2 md2,
              3 md3,
              4 md4

raid level = -1 linear mode
              0 striped mode
              other modes are only supported with persistent super blocks

chunk size factor = (raid-0 and raid-1 only)
              Set  the chunk size as 4k << n.
              
fault level = totally ignored
                            
dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
                            
A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:

e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro


Boot time autodetection of RAID arrays
--------------------------------------

When md is compiled into the kernel (not as module), partitions of
type 0xfd are scanned and automatically assembled into RAID arrays.
This autodetection may be suppressed with the kernel parameter
"raid=noautodetect".  As of kernel 2.6.9, only drives with a type 0
superblock can be autodetected and run at boot time.

The kernel parameter "raid=partitionable" (or "raid=part") means
that all auto-detected arrays are assembled as partitionable.

Boot time assembly of degraded/dirty arrays
-------------------------------------------

If a raid5 or raid6 array is both dirty and degraded, it could have
undetectable data corruption.  This is because the fact that it is
'dirty' means that the parity cannot be trusted, and the fact that it
is degraded means that some datablocks are missing and cannot reliably
be reconstructed (due to no parity).

For this reason, md will normally refuse to start such an array.  This
requires the sysadmin to take action to explicitly start the array
desipite possible corruption.  This is normally done with
   mdadm --assemble --force ....

This option is not really available if the array has the root
filesystem on it.  In order to support this booting from such an
array, md supports a module parameter "start_dirty_degraded" which,
when set to 1, bypassed the checks and will allows dirty degraded
arrays to be started.

So, to boot with a root filesystem of a dirty degraded raid[56], use

   md-mod.start_dirty_degraded=1


Superblock formats
------------------

The md driver can support a variety of different superblock formats.
Currently, it supports superblock formats "0.90.0" and the "md-1" format
introduced in the 2.5 development series.

The kernel will autodetect which format superblock is being used.

Superblock format '0' is treated differently to others for legacy
reasons - it is the original superblock format.


General Rules - apply for all superblock formats
------------------------------------------------

An array is 'created' by writing appropriate superblocks to all
devices.

It is 'assembled' by associating each of these devices with an
particular md virtual device.  Once it is completely assembled, it can
be accessed.

An array should be created by a user-space tool.  This will write
superblocks to all devices.  It will usually mark the array as
'unclean', or with some devices missing so that the kernel md driver
can create appropriate redundancy (copying in raid1, parity
calculation in raid4/5).

When an array is assembled, it is first initialized with the
SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
version number.  The major version number selects which superblock
format is to be used.  The minor number might be used to tune handling
of the format, such as suggesting where on each device to look for the
superblock.

Then each device is added using the ADD_NEW_DISK ioctl.  This
provides, in particular, a major and minor number identifying the
device to add.

The array is started with the RUN_ARRAY ioctl.

Once started, new devices can be added.  They should have an
appropriate superblock written to them, and then passed be in with
ADD_NEW_DISK.

Devices that have failed or are not yet active can be detached from an
array using HOT_REMOVE_DISK.


Specific Rules that apply to format-0 super block arrays, and
       arrays with no superblock (non-persistent).
-------------------------------------------------------------

An array can be 'created' by describing the array (level, chunksize
etc) in a SET_ARRAY_INFO ioctl.  This must has major_version==0 and
raid_disks != 0.

Then uninitialized devices can be added with ADD_NEW_DISK.  The
structure passed to ADD_NEW_DISK must specify the state of the device
and it's role in the array.

Once started with RUN_ARRAY, uninitialized spares can be added with
HOT_ADD_DISK.



MD devices in sysfs
-------------------
md devices appear in sysfs (/sys) as regular block devices,
e.g.
   /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.

  chunk_size
     This is the size if bytes for 'chunks' and is only relevant to
     raid levels that involve striping (1,4,5,6,10). The address space
     of the array is conceptually divided into chunks and consecutive
     chunks are striped onto neighbouring devices.
     The size should be atleast PAGE_SIZE (4k) and should be a power
     of 2.  This can only be set while assembling an array

  component_size
     For arrays with data redundancy (i.e. not raid0, linear, faulty,
     multipath), all components must be the same size - or at least
     there must a size that they all provide space for.  This is a key
     part or the geometry of the array.  It is measured in sectors
     and can be read from here.  Writing to this value may resize
     the array if the personality supports it (raid1, raid5, raid6),
     and if the component drives are large enough.

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