Raid-Dp And Double Parity; Internal Structure Of Raid-Dp - IBM N Series Hardware Manual

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Advantages of small RAID groups
Small RAID group configurations offer the following advantages:
Shorter disk reconstruction times
During disk failure within a small RAID group, data reconstruction time is usually shorter
than it would be within a large RAID group.
Decreased risk of data loss because of multiple disk failures
Data loss through double disk failure within a RAID 4 group is less likely than during a
triple disk failure within a RAID-DP group.

10.4 RAID-DP and double parity

It is well known that parity generally improves fault tolerance, and that single-parity RAID
improves data protection. Given that traditional single-parity RAID has a good track record to
date, the concept of double-parity RAID sounds like a better protection scheme. This is borne
out in the earlier example using the MTTDL formula. But what exactly is RAID-DP?
At the most basic layer, RAID-DP adds a second parity disk to each RAID group in a volume.
RAID group
A
group has data disks and one parity disk, with volumes that contain one or more RAID 4
groups. The parity disk in a RAID 4 volume stores row parity across the disks in a RAID 4
group. The additional RAID-DP parity disk stores diagonal parity across the disks in a
RAID-DP group (Figure 10-6). These two parity stripes in RAID-DP provide data protection in
the event of two disk failures that occur in the same RAID group.
Figure 10-6 RAID 4 and RAID-DP

10.4.1 Internal structure of RAID-DP

With RAID-DP, the traditional RAID 4 horizontal parity structure is still employed and becomes
a subset of the RAID-DP construct. In other words, how RAID 4 works on storage is not
modified with RAID-DP. Data is written out in horizontal rows with parity calculated for each
row in RAID-DP, which is considered the row component of double parity. If a single disk fails
or a read error from a bad block or bit error occurs, the row parity approach of RAID 4 is used
to re-create the data. RAID-DP is not engaged. In this case, the diagonal parity component of
RAID-DP is a protective envelope around the row parity component.
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IBM System Storage N series Hardware Guide
is an underlying construct on which volumes are built. Each traditional RAID 4

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