Troubleshooting
Problem
"Cache Hit Pct." variable shows a negative value (e.g -15%) when QUERY DB FORMAT=DETAILED
Cause
The variable "Total Buffer Latch Requests:" (in SHOW BUFS) shows an excessive value (e.g. 3845700982)
Resolving The Problem
Note: the use of the SHOW commands is unsupported. User to use on their discretion.
In some situations, running the query QUERY DB F=D shows the variable "Cache Hit Pct." as a negative value:
- Cache Hit Pct: -15.0
When a negative "Cache Hit Pct" is observed, acquire the "Total Buffer Latch Requests" by running the SHOW BUFS command. The output is similar to the following:
- Database Buffer Pool Statistics:
Total Buffer Latch Requests: 3845700982
Times Scavenging Required: 58122188 1.51%
Times Scavenging by Stealing: 58056652 1.51%
Times Scavenging by Waiting: 0 0.00%
Times Read Required: 57642282 1.50%
Cache Hit Percentage: -13.18%
This value is used in the calculation of the Cache Hit Pct., and since the value is huge, it overflows a 32 bit unsigned integer field, causing the negative result.
The resolution to this issue is to run the command: RESET BUFPOOL which will cause the buff pool statistics to be reset.
If the issue occurs again, then a schedule could be created to run this command frequently
depending on how often this issue occurs.
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Document Information
Modified date:
17 June 2018
UID
swg21268767