Re: [HACKERS] Sync vs. fsync during checkpoint

[Jan Wieck <JanWieck(at)Yahoo(dot)com>]

Bruce Momjian wrote:

> Jan Wieck wrote:
> > Tom Lane wrote:
> >
> > > "Zeugswetter Andreas SB SD" <ZeugswetterA(at)spardat(dot)at> writes:
> > >> So Imho the target should be to have not much IO open for the checkpoint, 
> > >> so the fsync is fast enough, even if serial.
> > > 
> > > The best we can do is push out dirty pages with write() via the bgwriter
> > > and hope that the kernel will see fit to write them before checkpoint
> > > time arrives.  I am not sure if that hope has basis in fact or if it's
> > > just wishful thinking.  Most likely, if it does have basis in fact it's
> > > because there is a standard syncer daemon forcing a sync() every thirty
> > > seconds.
> >
> > Looking at the response time charts I did for showing how vacuum delay 
> > is doing, it seems at least on Linux there is hope that that is the 
> > case. Those charts have just a regular 5 minute checkpoint with enough 
> > checkpoint segments for that, and no other sync effort done at all.
> >
> > The system has a hard time to handle a larger scaled test DB, so it is 
> > definitely well saturated with IO. The charts are here:
> >
> >      http://developer.postgresql.org/~wieck/vacuum_cost/
> >
> > > 
> > > That means that instead of an I/O storm every checkpoint interval,
> > > we get a smaller I/O storm every 30 seconds.  Not sure this is a big
> > > improvement.  Jan already found out that issuing very frequent sync()s
> > > isn't a win.
> >
> > In none of those charts I can see any checkpoint caused IO storm any 
> > more. Charts I'm currently doing for 7.4.1 show extremely clear spikes 
> > at checkpoints. If someone is interested in those as well I will put 
> > them up.
>
> So, Jan, are you basically saying that the background writer has solved
> the checkpoint I/O flood problem, and we just need to deal with changing
> sync to multiple fsync's at checkpoint?

ISTM that the background writer at least has the ability to lower the 
impact of a checkpoint significantly enough that one might not care 
about it any more. "Has the ability" means, it needs to be adjusted to 
the actual DB usage. The charts I produced where not done with the 
default settings, but rather after making the bgwriter a bit more 
agressive against dirty pages.

The whole sync() vs. fsync() discussion is in my opinion nonsense at 
this point. Without the ability to limit the amount of files to a 
reasonable number, by employing tablespaces in the form of larger 
container files, the risk of forcing excessive head movement is simply 
too high.


Jan

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