Pavan Deolasee wrote:
I have just counted the number of read/write calls on the CLOG blocks. As
you can
see the total number of CLOG reads jumped from 545323 to 1181851 i.e.
1181851 - 545323 = 636528 CLOG block reads for 1554697 pages of stock table.

Hmm. So there is some activity there. Could you modify the patch to count how many of those reads came from OS cache? I'm thinking of doing a gettimeofday() call before and after read, and counting how many calls finished in less than say < 1 ms. Also, summing up the total time spent in reads would be interesting.

Or, would it be possible to put the clog to a different drive, and use iostat to get the numbers?

This figure is only indicative since it also includes the CLOG block reads
which would
have happened as part of other backend operations (VACUUM took almost 55
minutes to
complete). Still in the first 210 minutes of the run, the total reads were
only 545323. So
most of the 636528 reads in the next 55 minutes can be attributed to VACUUM.

Actually, clog reads during normal activity is even worse.

  Heikki Linnakangas

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