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Re: [HACKERS] Re: Faster CREATE DATABASE by delaying fsync (was 8.4.1 ubuntu karmic slow createdb)

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org, pgsql-performance(at)postgresql(dot)org, Michael Clemmons <glassresistor(at)gmail(dot)com>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Subject: Re: [HACKERS] Re: Faster CREATE DATABASE by delaying fsync (was 8.4.1 ubuntu karmic slow createdb)
Date: 2010-02-02 17:43:15
Message-ID: 201002021843.49983.andres@anarazel.de (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Tuesday 02 February 2010 18:36:12 Robert Haas wrote:
> On Fri, Jan 29, 2010 at 1:56 PM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> > On Tue, Jan 19, 2010 at 3:25 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> That function *seriously* needs documentation, in particular the fact
> >> that it's a no-op on machines without the right kernel call.  The name
> >> you've chosen is very bad for those semantics.  I'd pick something
> >> else myself.  Maybe "pg_start_data_flush" or something like that?
> > 
> > I would like to make one token argument in favour of the name I
> > picked. If it doesn't convince I'll change it since we can always
> > revisit the API down the road.
> > 
> > I envision having two function calls, pg_fsync_start() and
> > pg_fsync_finish(). The latter will wait until the data synced in the
> > first call is actually synced. The fall-back if there's no
> > implementation of this would be for fsync_start() to be a noop (or
> > something unreliable like posix_fadvise) and fsync_finish() to just be
> > a regular fsync.
> > 
> > I think we can accomplish this with sync_file_range() but I need to
> > read up on how it actually works a bit more. In this case it doesn't
> > make a difference since when we call fsync_finish() it's going to be
> > for the entire file and nothing else will have been writing to these
> > files. But for wal writing and checkpointing it might have very
> > different performance characteristics.
> > 
> > The big objection to this is that then we don't really have an api for
> > FADV_DONT_NEED which is more about cache policy than about syncing to
> > disk. So for example a sequential scan might want to indicate that it
> > isn't planning on reading the buffers it's churning through but
> > doesn't want to force them to be written sooner than otherwise and is
> > never going to call fsync_finish().
> 
> I took a look at this patch today and I agree with Tom that
> pg_fsync_start() is a very confusing name.  I don't know what the
> right name is, but this doesn't fsync so I don't think it shuld have
> fsync in the name.  Maybe something like pg_advise_abandon() or
> pg_abandon_cache().  The current name is really wishful thinking:
> you're hoping that it will make the kernel start the fsync, but it
> might not.  I think pg_start_data_flush() is similarly optimistic.
What about: pg_fsync_prepare(). That gives the reason why were doing that and 
doesnt promise that it is actually doing an fsync.
I dislike really having "cache" in the name, because the primary aim is not to 
discard the cache...

Andres

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