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Status Functionality already exists
Workspace Db2
Components Monitoring
Created by Guest
Created on Jan 14, 2023

Display the prefetch wait time metrics in the monreport.dbsummary stored procedure .

The idea will be very useful in deciding the prefetcher performance in the database which helps in reducing the I/O usage in the system.

What is this prefetch wait time and when it occurs?(DB2LUW performance)

Prefetcheers are very important EDU's in the Db2 architecture which will pick the pages from the disk and place them in the bufferpools.
These pages are used by the applications for the processing.

Now imagine an agent is requesting for a page that has already been placed in a bufferpool by other agent then , agent has to wait untill that page task is done!. This waiting time of the agent is called as
"Prefetch wait time".

Some amount of prefetch wait time is very normal, since the reason a prefetch request was made was that the page was going to be needed by an agent shortly. Zero prefetch wait time is often a sign that prefetch isn’t happening at all, and that’s not always a good thing.

If prefetch wait time dominates, then that’s probably a sign that you should look into how prefetching is configured (number of prefetchers, extent size,prefetchsize) and at the configuration & performance of the storage system.

Metrics to calculate this prefetch wait time

1) Actual prefetch size in pages can be calculated from MON_GET_TABLESPACE


2)Average prefetch time estimation


The above estimation works as long as the following is true

pool_async_data_reads=(pool_data_p_reads + pool_temp_data_p_reads)

Needed By Not sure -- Just thought it was cool
  • Admin
    Ned Whelan
    Jun 21, 2023

    Please find the following documentation on the existing prefetch_wait_time monitor element:

  • Admin
    Ned Whelan
    May 24, 2023

    Thank you for submitting this Idea! Can you provide more information on exactly what is desired?