Tomas Vondra <tv@xxxxxxxx> writes: > I've been playing with various wal_level settings, mainly to find out > what is the overhead of hot standby, and I've noticed a strange thing. > In some cases the wal_level=minimal produces signigicantly more xlog > data than wal_level=archive (and hot_standby). > ... > Which means the pgbench init takes about 142kB with 'minimal' level and > about 163MB with 'archive' level (which is expected). > But the actual pgbench run produces much more xlog data with minimal wal > level compared to archive level. With minimal level it produces about > 90MB and with archive level it produces just about 5MB. > I've check the docs and the only possible explanation I've noticed when > reading the docs is that while wal_level=minimal allows to skip logging > of some bulk operations (e.g. COPY, which is exactly what pgbench does > when initializing the DB), it may need to log more data later (when > actually running the bench). 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? regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
