HP StorageWorks Command View SDM Installation and User Guide (T1086-96023, February 2005)

174 Command Line User Interface
RAID 5 DP Writes in Place OPAQUE The number of write operations to previously written LUN
addresses posted out of the write cache to RAID 5 DP
storage.
Usage
Rewrites are host writes to LUN addresses that have
already been written at least once since the LUN was
created. AutoRAID will decide whether to place write data
into RAID 1+0 or RAID 5 DP storage when it is posted out
of the write cache. When RAID 5 DP is the destination for
a rewrite, the data may be written in place in RAID 5 DP
by the conventional read, modify, write process or it may
be written by relocating the cluster containing the data.
This metric counts only those writes to RAID 5 DP that use
the read, modify, write process. Writes to RAID 5 DP that
use relocation are counted by the New RAID 5 DP Writes
metric. A relatively large number of RAID 5 DP writes in
place as compared to RAID 1+0 writes in place may be an
indication that random write performance is being limited
because there isn't enough disk space for the write
working set to fit in RAID 1+0 storage. In this case it may
be possible to improve random write performance by
adding more disks to the array.
Read Cache Hits ARRAY
LUN
The number of cache page accesses that were satisfied
immediately from data that was stored in the read or write
cache when processing host read commands. A host read
command that spans multiple read cache pages will count
a read cache hit for each read cache page that was
immediately available in the read cache when the read
command was processed. Any delays in accessing cache
pages due to read cache fetches, write cache posts or due
to address conflicts with data in the cache being read or
written to disk will not be counted as a read cache hit.
Usage
The ratio of Read Cache Hits to (Read Cache Hits + Cache
Pages Read) gives a general indication of the effectiveness
of the read cache in servicing the host read workload.
Effectiveness is approximately proportional to the ratio.
Table 12 Performance Metrics (Continued)
Metric Categories Description/Usage