Re: synchronous commit vs. hint bits

On Thursday, December 01, 2011 03:11:43 PM Robert Haas wrote:
> On Thu, Dec 1, 2011 at 4:09 AM, Andres Freund <andres@anarazel.de> wrote:
> >> Oh, that's interesting.  Why do you want to avoid frequent fsyncs?  I
> >> thought the point of synchronous_commit=off was to move the fsyncs to
> >> the background, but not necessarily to decrease the frequency.
> > 
> > Is that so? If it wouldn't avoid fsyncs how could you reach multiple
> > thousand TPS in a writing pgbench run on a pretty ordinary system with
> > fsync=on?
> Eh, well, what would stop you from achieving that?  An fsync operation
> that occurs in the background doesn't block further transactions from
> completing. 
But it will slow down overall system io. For one an fsync() on linux will 
cause a queue drain on the io submit queue. For another it counts against the 
total available random io ops a device can do.
Which in turn will cause slowdown for anything else doing syncronous random 
io. I.e. read(2).

> Meanwhile, getting the WAL records on disk faster allows
> us to set hint bits sooner, which is a significant win, as shown by
> the numbers I posted upthread.
Oh, that part I dont doubt. Sorry for that.


Andres