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Subject: ANNOUNCE: mdadm 3.0 - A tool for managing Soft RAID under Linux
I am pleased to (finally) announce the availability of
mdadm version 3.0
It is available at the usual places:
and via git at
This is a major new version and as such should be treated with some
caution. However it has seen substantial testing and is considerred
to be ready for wide use.
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
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 installed, 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
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.
NeilBrown 2nd June 2009