From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Tomas Vondra <tv(at)fuzzy(dot)cz> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: wal_level=minimal produces more data than archive level |
Date: | 2011-04-03 06:04:00 |
Message-ID: | 7425.1301810640@sss.pgh.pa.us |
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Lists: | pgsql-general |
Tomas Vondra <tv(at)fuzzy(dot)cz> writes:
> Dne 3.4.2011 02:45, Tom Lane napsal(a):
>> I wonder whether the discrepancy relates to having to produce
>> full-page-image WAL entries during the first touch of a page during the
>> pgbench run, versus having already done so in initialization. If you
>> force a checkpoint after the init step, do the results change?
> Yes, a forced CHECKPOINT results in a much more xlog data in case of the
> archive level. Without the checkpoint there was about 5MB, now there is
> about about 90MB (a fet kB more than with the minimal wal level).
> Hmmm, I'm wondering which of these two cases is more apropriate when
> comparing wal levels - with the checkpoint or without it? I believe the
> one with checkpoint, as checkpoints happen all the time anyway.
Well, both of them are boundary cases --- in a realistic situation you'd
have some but not all pages already dirty since the last checkpoint.
The important point here is that you had non-comparable starting
conditions.
regards, tom lane
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