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

182 Command Line User Interface
Write Cache Hits ARRAY
LUN
The number of cache page accesses that were satisfied
immediately from data that was stored in the write cache
when processing host write commands. A host write
command that spans multiple write cache pages will count
a write cache hit for each write cache page that was
immediately available in the write cache when the write
command was processed. Any delays in accessing cache
pages due to 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 write cache hit. A cache page access
that causes the allocation of a new write cache page from
the pool of free write cache pages without waiting for a
write cache post to complete is counted as a write cache
hit.
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.
Table 12 Performance Metrics (Continued)
Metric Categories Description/Usage