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

152 Command Line User Interface
Cache Pages Written ARRAY
LUN
The number of write cache pages written to disk in
response to host write commands. Each write cache page
is counted as a single cache page written independent of
the number of disk write commands used to write the
page. Disk writes generated by array internal policy
operation are not counted as cache pages written.
Usage
The ratio of Write Cache Hits to Cache Pages Written
gives a general indication of the effectiveness of the write
cache in servicing the host write workload. The write
cache serves primarily as a buffer to absorb bursts of high
write activity in the host workload and provide a low
latency response then write the data to disk during a time
of lower activity. All data that comes into the write cache
will eventually be written to disk. If this ratio is near one
that is an indication that there is an approximate one to
one correspondence between incoming and outgoing
writes. In that case a high percentage of the incoming
writes will have a low latency response. If this ratio is
larger than one the write cache is providing an additional
benefit of combining multiple incoming writes into a single
disk write. If this ratio is less than one a percentage of
incoming writes related to the value of the ratio will not be
receiving a low latency response.
Clusters Relocated ARRAY
OPAQUE
The number of clusters relocated.
Usage
There are a number of different processes that can cause
data migration both in foreground and background. This
metric is an over all measure of the amount of work being
performed for migration. Some migration activity is only
performed in background as idle time is available and so
does not interfere with processing of the host workload.
Other migration activity is performed in foreground and
consumes resources that might otherwise be used to
service the host workload. This metric does not provide a
means to distinguish the type of migration taking place
and so is not a reliable indicator of the performance
impact of migration.
Table 12 Performance Metrics (Continued)
Metric Categories Description/Usage