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From: Neil Brown <neilb <at> suse.de>
Subject: ANNOUNCE: mdadm 3.0-devel3 - A tool for managing Soft RAID under Linux
Newsgroups: gmane.linux.raid
Date: Tuesday 10th March 2009 06:02:45 UTC (over 8 years ago)
I am pleased to announce the availability of
   mdadm version 3.0-devel3

It is available at the usual places:
and via git at

Note that this is a "devel" release.  It should be used with
caution, though it is believed to be close to release-candidate stage.

There have been numerous improvements and additions since -devel2.
I think we are close to a release of 3.0.

I need to add lots of tests to the test suite to test the new
functionality.  And I need to review the man pages.

After that I will release -rc1 followed by -final.

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

Any testing and feedback will be greatly appreciated.

NeilBrown  10th March 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

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.
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