excellent explanation from
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http://www.backupcentral.com/high-availability.html <code>
The RAID ?hierarchy? begins with RAID 0 (striping) and RAID 1
(mirroring). Combining RAID 0 and RAID 1 is called RAID-0+1 or
RAID-1+0, depending on how you combine them. (RAID 0+1 is also called
RAID-01, and RAID-1+0 is also called RAID-10.) The performance of
RAID-10 and RAID-01 are identical, but they have different levels of
data integrity.
RAID-01 (or RAID 0+1) is a mirrored pair (RAID-1) made from two stripe
sets (RAID-0); hence the name RAID 0+1, because it is created by first
creating two RAID-0 sets and adding RAID-1. If you lose a drive on one
side of a RAID-01 array, then lose another drive on the other side of
that array before the first side is recovered, you will suffer complete
data loss. It is also important to note that all drives in the
surviving mirror are involved in rebuilding the entire damaged stripe
set, even if only a single drive was damaged. Performance during
recovery is severely degraded during recovery unless the RAID subsystem
allows adjusting the priority of recovery. However, shifting the
priority toward production will lengthen recovery time and increase the
risk of the kind of the catastrophic data loss mentioned earlier.
RAID-10 (or RAID 1+0) is a stripe set made up from N mirrored pairs.
Only the loss of both drives in the same mirrored pair can result in any
data loss and the loss of that particular drive is 1/Nth as likely as
the loss of some drive on the opposite mirror in RAID-01. Recovery only
involves the replacement drive and its mirror so the rest of the array
performs at 100% capacity during recovery. Also since only the single
drive needs recovery bandwidth requirements during recovery are lower
and recovery takes far less time reducing the risk of catastrophic data
loss.