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Título
What are the DB2 Overall Cache Hit Ratio and Index Cache Hit Ratio alarms used for? -
Descrição
How are the following alarms calculated and what are they used for?
- DB2 - Member Index Cache Hit Ratio
- DB2 - Member Overall Cache Hit Ratio
- DB2 - Database Index Cache Hit Ratio
- DB2 - Database Overall Cache Hit Ratio
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Causa
These alarms will be fired when the corresponding cache hit ratio of the database's or member's I/O activity falls below a predefined threshold.
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Resolução
Overall Cache Hit Ratio
This is overall hit ratio of logical reads to physical reads, namely: the percentage of database page I/O requests that were satisfied from the buffer cache and therefore did not have to perform disk reads. This ratio measures how efficiently DB2 is using the memory allocated to its buffer cache.
The overall cache hit ratio can be calculated using the following formula:(1 - physical_reads/(logical_reads + 1))*100
- Low values = more physical IO
- High values = more logical IO
- 85-90% is a good starting point
The Database Overall Cache Hit Ratio value can be reflected in the Databases Overview screen under Memory | Buffer Pool
If this alarm fires, consider investigating the related pool activity and increasing the bufferpool size in the DB2 instance.
Index Cache Hit Ratio
When either the "DB2 - Member Index Cache Hit Ratio" or "DB2 - Database Index Cache Hit Ratio" alarms fire, the index related pool activity should be investigated to identify the root cause of the low index hit ratio.
The index cache hit ratio can be calculated using the following formula:
(1 - POOL_INDEX_P_READS/(POOL_INDEX_L_READS + 1))*100
- Low values = more physical IO
- High values = more logical IO
- 85-90% is a good starting point
One consideration in resolving this issue is to enlarge the bufferpool size.
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Anexos
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Informações adicionais
The attached document provides more information on DB2 bufferpool tuning.