HP StorageWorks 7110 - Virtual Array, Service Manual, Page: 49 / 164

Product Overview 49
Product Over
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simultaneous disk failures. The two redundancy segments are referred to as
“P” and “Q” parity. P, like traditional RAID 5 arrays, uses an XOR (parity)
algorithm. P parity is based on Reed-Solomon ECC technology, similar to
error detection and correction found in ECC DRAM.
Application data, and the P and Q parity data, rotate to different disks for
each stripe in a RAID Group. Like RAID 1+0, this effectively eliminates hot
spots.
A read operation only requires a single access to the disk(s) containing the
data, a small (<256 Kbytes) write operation requires that the data, and the P
and Q parity data be updated – this is the source of the small random write
performance impact. For larger (>256 Kbytes) write operations, the Virtual
Array implements a log-structured RAID 5DP write. Log-structured writes
effectively eliminate the read-modify-write associated with small block writes to
RAID 5DP by redirected the write operation to a new RAID 5DP stripe. The P
and Q parity data is held in non-volatile write cache until the whole stripe is
written, then the P and Q are written. Thus the P and the Q are written only
once for each stripe.
Note Until a rebuild is complete, the array is operating in a degraded
mode. In degraded mode, the array will use P and/or Q parity
to reconstruct data that resided on the failed disk.
Figure 19 is an example showing the distribution of user data and parity data
in a RAID 5DP configuration. The example shows one RAID group with five
stripes: three data segments and two parity segments (P and Q). The
segments are striped across the disks in a rotating fashion. Note that any two
disks can fail, but the data, P, or the Q parity is always available to complete a
read operation.