On 2016-02-13 11:10:58 -0500, Tom Lane wrote: > Magnus Hagander <mag...@hagander.net> writes: > > The big thing is, IIRC, that we turn off the optimizations in > > min_wal_level. *most* people will see no impact of their regular > > application runtime from that, but it might definitely have an effect on > > data loads and such. For normal runtime, there should be very close to zero > > difference, no? > > I was asking for a demonstration of that, not just handwaving. Even if > it was measured years ago, I wouldn't assume the comparison would be > the same on current Postgres.
Well, let's look at what the difference between wal_level's are: 1) the (currently broken) optimization of not WAL logging COPY et al, for newly created relations. 2) relation AccessExclusiveLocks are WAL logged on >= hot_standby 3) Subtransaction assignment records are generated for >= hot_standby after 64 records. 4) checkpoints and bgwriter occasionally generate XLOG_RUNNING_XACTS records 5) btreevacuum() and _bt_getbuf() sometimes do additional WAL logging on >= hot_standby 6) Once per vacuum we issue a XLOG_HEAP2_CLEANUP_INFO 1) obviously can have performance impact; but only in a relatively narrow set of cases. I doubt any of the others really play that major a role. But I really think minor efficiency differences are besides the point. Making backups and replication easier has a far bigger positive impact, for far more users. - Andres -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers