Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> Those things are related, though. Freezing sooner could be
>> viewed as an alternative to hint bits.
>
> Freezing sooner isn't likely to reduce I/O compared to hint bits.
> What that does is create I/O that you *have* to execute ... both
> in the pages themselves, and in WAL.
In an environment where the vast majority of tuples live long enough
to need to be frozen anyway, freezing sooner doesn't really do that
to you. Granted, explicit freezing off-hours prevents autovacuum
from doing that to you in large bursts at unexpected times, but if
you're comparing background writer freezing to autovacuum freezing,
I'm not clear on where the extra pain comes from.
I am assuming that the background writer would be sane about how it
did this, of course. We could all set up straw man implementations
which would clobber performance. I suspect that you can envision a
hueristic which would be no more bothersome than autovacuum.
-Kevin