IBM System Storage DS4000, Руководство по оборудованию

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Appendix A. Overview of IBM System Storage DS5000 RAID types
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Table A-3 RAID levels comparison
RAID reliability considerations
At first glance, both RAID 3 and RAID 5 appear to provide good protection against drive
failure. With today’s high-reliability drives, it appears unlikely that a second drive in an array
will fail (causing data loss) before an initial failed drive can be replaced. But if you look at
RAID 6 and calculate the possibility of data loss, the chance to loose data is theoretically
much less than on RAID 3 and RAID 5.
However, field experience has shown that when a RAID 3 or RAID 5 array fails, it is not
usually due to two drives in the array experiencing complete failure. Instead, most failures are
caused by one drive going bad, and a single block somewhere else in the array that cannot be
read reliably.
RAID Description Application Advantage Disadvantage
0 Stripes data across
multiple drives. IOPS
Mbps Performance, due to
parallel operation of the
access. No redundancy. If one
drive fails, the data is
lost.
1 The disk's data is
mirrored to another
drive. IOPS Performance, as
multiple requests can
be fulfilled
simultaneously. Storage costs are
doubled.
10 Data is striped across
multiple drives and
mirrored to the same
number of disks. IOPS Performance, as
multiple requests can
be fulfilled
simultaneously with
better data protection. Storage costs are
doubled.
3 Drives operate
independently with
data blocks
distributed among all
drives. Parity is
written to a dedicated
drive. Mbps High performance for
large, sequentially
accessed files (image,
video, and graphics). Degraded
performance with 8-9
I/O threads, random
IOPS, and smaller,
more numerous
IOPS.
5 Drives operate
independently with
data and parity blocks
distributed across all
drives in the group. IOPS
Mbps Good for reads, small
IOPS, many concurrent
IOPS, and random I/Os. Writes are particularly
demanding.
6 Stripes blocks of data
and parity across an
array of drives and
calculates two sets of
parity information for
each block of data. IOPS
Mbps Good for multi-user
environments, such as
database or file system
storage, where typical
I/O size is small, and in
situations where
additional fault
tolerance is required.
This is the most reliable
RAID level on the
DS5000 storage
subsystem Slower in writing data,
complex RAID
controller
architecture.