> Do we know why anti-wraparound uses so many resources in the first place?
>  The default settings seem to be quite conservative to me, even for a
> system that has only a single 5400 rpm hdd (and even more so for any real
> production system that would be used for a many-GB database).
> I wonder if there is something simple but currently unknown going on which
> is causing it to damage performance out of all proportion to the resources
> it ought to be using.

Does anti-wraparound vacuum (AWAV) write synchronously?  If so, there's
a potential whole world of hurt there.

Otherwise, the effect you're seeing is just blowing out various caches:
the CPU cache, storage cache, and filesystem cache.  While we can (and
do) prevent vacuum from blowing out shared_buffers, we can't do much
about the others.

Also, locking while it does its work.

Josh Berkus
PostgreSQL Experts Inc.

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