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Re: synchronous commit vs. hint bits

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, YAMAMOTO Takashi <yamt(at)mwd(dot)biglobe(dot)ne(dot)jp>, simon(at)2ndquadrant(dot)com
Subject: Re: synchronous commit vs. hint bits
Date: 2011-12-01 14:18:29
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thursday, December 01, 2011 03:11:43 PM Robert Haas wrote:
> On Thu, Dec 1, 2011 at 4:09 AM, Andres Freund <andres(at)anarazel(dot)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.


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