DASD Cache Analysis
Specifics - DASD Cache analysis:
Cache is used in the DASD control units to keep data in 'memory' until actually written to disk. Cache is a very effective
way for a DASD control unit to keep I/O moving quickly and efficiently. If cache is unavailable, each write has to be
done in the foreground instead of the background and the system performance can grind to a halt. Watch for:
- Cache fast write is active. It is very important for fast write to be active.
- The channel paths are online.
- The cache hit ratio is high.
- NVS is zero - if NVS (Non-Volatile Storage) is full, cache to no longer being utilized.
- Cache Inhibit/Bypass are zeros - if not, cache is not being utilized.
- Keep in mind that the data in the screens/reports are from all LPARS connected to the control units.
ESADSDC - Shows the current cache control unit configuration and characteristics.
Control Unit - This shows the device number and model number.
Storage Director - This shows the storage director id and its status. Verify all are online.
Available Cache - This shows the available cache. It may be less than the total size if other LPARs are
also using that control unit.
Cache Fast Write - This shows that cache fast write is active. Active is good.
Channel Paths Online - This shows the channel paths online to each device. Verify each expected device is
online.
ESADSD5 - Shows the cache control unit performance.
Device Number - This shows the device number and model number for the head of string.
Can click (zVIEW) or zoom (z/VM) to see the all of the devices on the string.
Total I/O Cache Hit% - This shows how well cache is being utilized. A low hit% (below 80) may indicate
more cache is needed or it might be batch (like backups).
NVS Full - This indicates the Non-volatile storage is full. This stops fast write (caching), which is a
big problem as it will cause the disks to be more highly utilized and slow down.
This is a good place for an alert.
Cache Inhib/Bypass - This also will indicate that caching is not working.
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