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there will be a synchronization phase as soon as the RAID device is created. After some time
(the exact amount will depend on the actual size of the disks…), the RAID array switches to the
“active” state. Note that during this reconstruction phase, the mirror is in a degraded mode,
and redundancy isn't assured. A disk failing during that risk window could lead to losing all the
data. Large amounts of critical data, however, are rarely stored on a freshly created RAID array
before its initial synchronization. Note that even in degraded mode, the /dev/md1 is usable,
and a filesystem can be created on it, as well as some data copied on it.
TIP
Starting a mirror in degraded
mode
Sometimes two disks are not immediately available when one wants to start
a RAID-1 mirror, for instance because one of the disks one plans to include
is already used to store the data one wants to move to the array. In such
circumstances, it is possible to deliberately create a degraded RAID-1 array
by passing missing instead of a device file as one of the arguments to mdadm.
Once the data have been copied to the “mirror”, the old disk can be added to
the array. A synchronization will then take place, giving us the redundancy
that was wanted in the first place.
TIP
Seing up a mirror without
synchronization
RAID-1 volumes are oen created to be used as a new disk, oen considered
blank. The actual initial contents of the disk is therefore not very relevant,
since one only needs to know that the data wrien aer the creation of the
volume, in particular the filesystem, can be accessed later.
One might therefore wonder about the point of synchronizing both disks at
creation time. Why care whether the contents are identical on zones of the
volume that we know will only be read aer we have wrien to them?
Fortunately, this synchronization phase can be avoided by passing the --
assume-clean option to mdadm. However, this option can lead to surprises in
cases where the initial data will be read (for instance if a filesystem is already
present on the physical disks), which is why it isn't enabled by default.
Now let's see what happens when one of the elements of the RAID-1 array fails. mdadm, in particular
its --fail option, allows simulating such a disk failure:
# mdadm /dev/md1 --fail /dev/sdh
mdadm: set /dev/sdh faulty in /dev/md1
# mdadm --detail /dev/md1
/dev/md1:
[...]
Update Time : Thu Sep 30 15:45:50 2010
State : active, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 1
Spare Devices : 0
Name : squeeze:1
(local to host squeeze)
Chapter 12 — Advanced Administration
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