Kbase P4317: Record Reads, Database Reads, Logical Reads and O/S Reads.
| Autor |
Progress Software Corporation - Progress |
| Acesso |
Público |
| Publicação |
3/12/2003 |
|
Status: Unverified
GOAL:
Under promon what is the relationship between Record Reads, Database Reads, Logical Reads and O/S Reads?
FIX:
Looking under promon option 5. Activity, values are seen for Record Reads and DB Reads.
Record Reads are the number of records read, while DB Reads are the number of database blocks read from disk.
The value for DB Reads is expressed as number of blocks, and the value for record reads is for single records, so since there can be various numbers of records per block, the buffer hit rate involves an algorithm which does conversions between the two.
Looking under promon R&D options, Activity Displays > Buffer Cache, values are seen for Logical Reads and O/S Reads.
Logical Reads are the total number of client requests for database block read operations.
O/S Reads is the same concept as the DB Reads seen under promon option 5. Activity, i.e. the number of db blocks read from disk rather than from the database buffers.
The hitrate on the blocks logical/physical reads is actually what the buffer pool hitrate is (what you see as the Buffer Hits under promon option 5. Activity, and Buffer pool hit rate under R&D > Other Displays > Performance Indicators). The approximate algorithm for Buffer Hits is:
Buffer Hits ~= ((Logical reads - O/S reads) * 100)/Logical reads
This value is a measure of how often Progress found a record in the buffer rather than having to retrieve it from disk. A value of 85-95% is good: below 85% and there aren't enough buffers allocated with the -B parameter; above 95% then there are probably too many allocated.