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again due to having been probed during the reboot. The kernel would then have three physical
elements, each claiming to contain half of the same RAID volume. Another source of confusion
can come when RAID volumes from two servers are consolidated onto one server only. If these
arrays were running normally before the disks were moved, the kernel would be able to detect
and reassemble the pairs properly; but if the moved disks had been aggregated into an md1 on
the old server, and the new server already has an md1, one of the mirrors would be renamed.
Backing up the configuration is therefore important, if only for reference. The standard way to
do it is by editing the /etc/mdadm/mdadm.conf file, an example of which is listed here:
# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#
# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
DEVICE /dev/sd*
# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes
# automatically tag new arrays as belonging to the local system
HOMEHOST
# instruct the monitoring daemon where to send mail alerts
MAILADDR root
ARRAY /dev/md0 metadata=1.2 name=squeeze:0 UUID=6194b63f:69a40eb5:a79b7ad3:
➥ c91f20ee
ARRAY /dev/md1 metadata=1.2 name=squeeze:1 UUID=20a8419b:41612750:b9171cfe:00
➥ d9a432
Example 12.1
mdadm configuration file
One of the most useful details is the DEVICE option, which lists the devices where the system
will automatically look for components of RAID volumes at start-up time. In our example, we
replaced the default value, partitions, with an explicit list of device files, since we chose to use
entire disks and not only partitions, for some volumes.
The last two lines in our example are those allowing the kernel to safely pick which volume
number to assign to which array. The metadata stored on the disks themselves are enough to
re-assemble the volumes, but not to determine the volume number (and the matching /dev/md*
device name).
308 The Debian Administrator's Handbook