Commit a3a173cd authored by Neil Brown's avatar Neil Brown Committed by Linus Torvalds

[PATCH] md: Allow md to select between superblock formats

The code to understand a specific superblock format is
already highly localised in md.  This patch defines a
user-space interface for selecting which superblock format
to use, and obeys that selection.

Md currently has a concept of 3 version numbers:
 A major version number
 A minor version number
 A patch version number

There historically seems to be some confusion about whether
these refer to a version of the superblock layout,
or a version of the software.

We will now define that:
   the "major_version" defines the superblock handler.
      '0' is the current superblock format.  All new formats
          will need new numbers.
   the "minor_version" can specify minor variations in the
      superblock, such as different location on the device
   the "patch_version" will be used to indicate new extenstions
      to the software.. patch_version=1 will mean multiple superblock
      support.

A superblock version number is selected by specifing major_version
in SET_ARRAY_INFO ioctl.

This patch:
  Updates Documentation/md.txt with details of new interface.
  Generalises desc_nr handling and makes sure that an array never
  has two devices with the same desc_nr.
  makes sure mddev->major_version is always valid and is 0 by default.
  uses mddev->major_version to select superblock handlers.
  Modifies set_array_info to just record version number if raid_disks==0
  Makes sure max_disks is always set correctly.
  Determines device size when reading superblock, or a hot-add/add-new.
parent b931e459
Tools that manage md devices can be found at
http://www.<country>.kernel.org/pub/linux/daemons/raid/....
http://www.<country>.kernel.org/pub/linux/utils/raid/....
You can boot (if you selected boot support in the configuration) with your md
device with the following kernel command lines:
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
......@@ -33,4 +33,64 @@ 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
-------------------------------
The md driver can support a variety of different superblock formats.
(It doesn't yet, but it can)
The kernel does *NOT* autodetect which format superblock is being
used. It must be told.
Superblock format '0' is treated differently to others for legacy
reasons.
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 approrpriate redundancy (copying in raid1, parity
calculation in raid4/5).
When an array is assembled, it is first initialised 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-presistant).
-------------------------------------------------------------
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 uninitialised 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.
One started with RUN_ARRAY, uninitialised spares can be added with
HOT_ADD_DISK.
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