Re: wal_level=minimal produces more data than archive level

Tomas Vondra <tv@fuzzy.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