What does it mean? What do I do about it?
Example alert from the ProTop Portal:
What does it mean?
The "Hit Ratio" is the percentage of time a data block is found in memory rather than fetched from disk. A 99% hit ratio means 1 in 100 record reads requires a disk IO. 98% means 1 in 50, twice as bad!
A change in the hit ratio that seems small can greatly impact performance. As a rule of thumb - a hit ratio of less than 95% is awful, 98% is barely tolerable, and 99% is starting to be ok.
A low hit rate can be caused by a high volume of reads made by a report running. This happening during business hours when your users are most active is much worse than during a backup running in the early morning hours or month-end processing running over a weekend.
What to do?
Decrease unnecessary high-volume reads. And consider increasing your -B to use as much available memory as possible.
NOTE: Large changes in -B are required to improve Hit Ratio significantly. For example, you must increase -B by approximately 4x to reduce disk IO operations by half.
This is NOT a recommendation! Do not increase -B by 4x unless you understand that that is exactly what you need to do!
See Tuning your database buffer pool for more information.
If all else fails...
Contact us at support@wss.com or use the online chat. We'll be happy to help.