SCF Reference Manual for the Storage Subsystem (G06.24+, H06.03+)

Displaying Information About Magnetic Disks
SCF Reference Manual for the Storage Subsystem—529937-007
5-17
Reconfiguring Cache to Resolve Performance
Problems
Reconfiguring Cache to Resolve Performance Problems
Based on the statistics displayed by a STATS DISK report, you can reconfigure cache
to resolve memory performance problems.
Cache Write
Cleans
the percentage of cache writes for which the block was found
unchanged in cache (not dirty). When new data is inserted into a
block, the disk process does not have to perform a disk read but
it must perform a disk write. This counter is also incremented
when an end of file (EOF) is extended (an application appends to
the end of a file).
Cache Write
Misses
the percentage of cache writes that resulted in the disk process
not finding a block and having to read the block from the disk.
For cache write misses, the disk process must perform both a
disk read and a disk write. (When added together, the
percentages displayed for Cache Write Cleans, Cache Write
Misses, and Cache Write Dirties equal 100 percent.)
Cache Calls the total number of cache reads and cache writes performed
during the measurement interval. If a user request bypasses
cache (for example, by specifying direct I/O), none of the cache
counters are affected by that call.
Cache Faults the number of times a cache call expected to find the block in
cache but could not, due to limited memory space. This number
should be very small or zero. If it is not, there is insufficient
memory to satisfy the cache configuration. You must either add
more memory or reconfigure cache to use memory more
efficiently.
Audit Forces the number of times an audited dirty block required an audit-trail
write to disk to make room for a new block to be read from disk.
(This value is the same as the AUDIT-BUF-FORCE counter in the
disk report from the Measure product.)
Condition What to Do
Cache Read Hits is low.
Cache Read Misses is high.
Use the ALTER DISK, CACHE command to increase the
number of cache blocks for that cache block size.
Cache Faults is large. Use the ALTER DISK, CACHE command to reduce the
number of cache blocks for that cache block size.
Audit Forces is high (indicates
insufficient cache memory and
tends to take resources from
other processes).
If the processor that controls the disk you are analyzing
has enough available physical memory, use the ALTER
DISK, CACHE command to allocate more memory to the
cache (increase the number of blocks) for that cache
block size.
Cache Faults is not close to
zero.
Add more memory or reconfigure cache to use less
memory.