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drives and can balance reads across both drives (because there are two copies). This can be
coupled with RAID 0 to form RAID 1+0 (or RAID 10), which mirrors a stripe set, or to form
RAID 0+1, which stripes data across pairs of mirrors. This has the benei t of being able to
withstand multiple drives failing, but only if the drives fail on different elements of a stripe
on different mirrors, thus making RAID 1+0 more fault tolerant than RAID 0+1. The other
benei t of a mirrored RAID coni guration is that, in the case of a failed drive, rebuild times
can be very rapid, which shortens periods of exposure.
Figure 6.3
h is R AID 10 2+2
confi guration
provides good per-
formance and good
availability, but at
the cost of 50 per-
cent of the usable
capacity.
Data: 1011
Write: 10
Write: 11
Write: 10
Write: 11
Parity RAID (RAID 5, RAID 6) These RAID levels use a mathematical calculation (an XOR
parity calculation) to represent the data across several drives. This tends to be a good compro-
mise between the availability of RAID 1 and the capacity efi ciency of RAID 0. RAID 5 calcu-
lates the parity across the drives in the set and writes the parity to another drive. This parity
block calculation with RAID 5 is rotated among the arrays in the RAID 5 set.
Parity RAID schemes can deliver very good performance, but there is always some degree of
write penalty. For a full-stripe write, the only penalty is the parity calculation and the parity
write, but in a partial-stripe write, the old block contents need to be read, a new parity calcu-
lation needs to be made, and all the blocks need to be updated. However, generally modern
arrays have various methods to minimize this effect.
Read performance, on the other hand, is generally excellent because a larger number of
drives can be read from than with mirrored RAID schemes. RAID 5 nomenclature refers
to the number of drives in the RAID group, so Figure 6.4 would be referred to as a RAID 5
4+1 set. In the i gure, the storage efi ciency (in terms of usable to raw capacity) is 80 percent,
which is much better than RAID 1 or 10.
Figure 6.4
A R AID 5 4+1 con-
fi guration off ers a
balance between
performance and
e ciency.
Data: 1011
Write: 1
Write: 0
Write: 1
Write: 1
Write: Parity
RAID 5 can be coupled with stripes, so RAID 50 is a pair of RAID 5 sets with data striped
across them.
When a drive fails in a RAID 5 set, I/O can be fuli lled using the remaining drives and the
parity drive, and when the failed drive is replaced, the data can be reconstructed using the
remaining data and parity.
 
 
 
 
 
 
 
 
 
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