Re: vacuum, performance, and MVCC

Ühel kenal päeval, R, 2006-06-23 kell 10:30, kirjutas Mark Woodward:

> >> > What is interesting is setting up the server so that you
> >> > can service your loads comfortably. Running the server at 100% lead is
> >> > not anything you want to do on production server. There will be things
> >> > you need to do anyway and you need some headroom for that.
> >>
> >> Of course, you design it so peaks are easily managed, but unless you run
> >> vacuum continuously, and that has its own set of problems, you run into
> >> this problem, and it can get really really bad.
> >
> > Usually it gets really bad if you *don't* run vacuum continuously, maybe
> > hopeing to do it in slower times at night. For high-update db you have
> > to run it continuously, maybe having some 5-15 sec pauses between runs.
>
> And how much I/O does this take?

Surprisingly its mostly WAL traffic, the heap/index pages themselves are
often not yet synced to disk by time of vacuum, so no additional traffic
there. If you had made 5 updates per page and then vacuum it, then you
make effectively 1 extra WAL write meaning 20% increase in WAL traffic.

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