main: factor out code to parse layout for raid10 and faulty.

[thirdparty/mdadm.git] / ANNOUNCE-3.0-rc1

Subject:  ANNOUNCE: mdadm 3.0-rc1 - A tool for managing Soft RAID under Linux

I am pleased to announce the availability of
   mdadm version 3.0-rc1

It is available at the usual places:
   countrycode=xx.
   http://www.${countrycode}kernel.org/pub/linux/utils/raid/mdadm/
and via git at
   git://neil.brown.name/mdadm
   http://neil.brown.name/git?p=mdadm

This is a "release candidate" which means that I think it is safe
to use and that there will be no significant change in functionality
before release.

The man pages aren't really "release candidate" yet but I will be
working on them before the final release.

The most significant changes since -devel3 relate to the names of md
devices as they appear in /dev and /dev/md/, and in particular the names
that are used when an array is assembled with "--incremental" or with
"mdadm --assemble --scan" when there are no ARRAY lines in mdadm.conf.
In these cases mdadm needs to deduce a name to use, and to try to
avoid using a name that a different array might have a stronger claim to.
The rules are:
  - if the array is mentioned in mdadm.conf, use the name given there.
  - if the array appear to have been created for "this host" using the
    "homehost" concept, trust the name given in the metadata
  - if the new setting "HOMEHOST <ignore>" is given (can be in mdadm.conf
    or on command line) the the name given in the metadata is not
    associated with some other array by mdadm.conf, then trust the
    name given in the metadata
  - otherwise use the name in the metadata, but in an untrusted manner.

If a name is untrusted, or if the name is already in use by another
array, then a numeric suffix like "_0", "_1" is appended to create
a unique name for the array.

That name is then used to create a device file in /dev/md/.

So if all arrays needed for boot will always be listed in
/etc/mdadm.conf, then it is appropriate to add "HOMEHOST <ignore>" to
mdadm.conf and there is no risk of conflicting names.  However if you
want auto-assemble to assemble all arrays at boot time and you don't
want to list them in mdadm.conf, then don't give "HOMEHOST <ignore>"
either else there could be a risk of the wrong array being assembled
for a given name.



The following is the same introduction to 3.x as appeared in
previous announcements.

Any testing and feedback will be greatly appreciated.

NeilBrown  11th May 2009


=====================================================

The significant change which justifies the new major version number is
that mdadm can now handle metadata updates entirely in userspace.
This allows mdadm to support metadata formats that the kernel knows
nothing about.

Currently two such metadata formats are supported:
  - DDF  - The SNIA standard format
  - Intel Matrix - The metadata used by recent Intel ICH controlers.

Also the approach to device names has changed significantly.

If udev is installed on the system, mdadm will not create any devices
in /dev.  Rather it allows udev to manage those devices.  For this to work
as expected, the included udev rules file should be installed.

If udev is not install, mdadm will still create devices and symlinks 
as required, and will also remove them when the array is stopped.

mdadm now requires all devices which do not have a standard name (mdX
or md_dX) to live in the directory /dev/md/.  Names in this directory
will always be created as symlinks back to the standard name in /dev.

The man pages contain some information about the new externally managed
metadata.  However see below for a more condensed overview.

Externally managed metadata introduces the concept of a 'container'.
A container is a collection of (normally) physical devices which have
a common set of metadata.  A container is assembled as an md array, but
is left 'inactive'.

A container can contain one or more data arrays.  These are composed from
slices (partitions?) of various devices in the container.

For example, a 5 devices DDF set can container a RAID1 using the first
half of two devices, a RAID0 using the first half of the remain 3 devices,
and a RAID5 over thte second half of all 5 devices.

A container can be created with

   mdadm --create /dev/md0 -e ddf -n5 /dev/sd[abcde]

or "-e imsm" to use the Intel Matrix Storage Manager.

An array can be created within a container either by giving the
container name and the only member:

   mdadm -C /dev/md1 --level raid1 -n 2 /dev/md0

or by listing the component devices

   mdadm -C /dev/md2 --level raid0 -n 3 /dev/sd[cde]

To assemble a container, it is easiest just to pass each device in turn to 
mdadm -I

  for i in /dev/sd[abcde]
  do mdadm -I $i
  done

This will assemble the container and the components.

Alternately the container can be assembled explicitly

   mdadm -A /dev/md0 /dev/sd[abcde]

Then the components can all be assembled with

   mdadm -I /dev/md0

For each container, mdadm will start a program called "mdmon" which will
monitor the array and effect any metadata updates needed.  The array is
initially assembled readonly. It is up to "mdmon" to mark the metadata 
as 'dirty' and which the array to 'read-write'.

The version 0.90 and 1.x metadata formats supported by previous
versions for mdadm are still supported and the kernel still performs
the same updates it use to.  The new 'mdmon' approach is only used for
newly introduced metadata types.