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

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)anarazel(dot)de>, 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: 2010-02-03 13:42:57
Message-ID: 603c8f071002030542r779a4f47k78355c62b0b7853@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Wed, Feb 3, 2010 at 6:53 AM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> On Tue, Feb 2, 2010 at 7:45 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I think you're probably right, but it's not clear what the new name
>> should be until we have a comment explaining what the function is
>> responsible for.
>
> So I wrote some comments but wasn't going to repost the patch with the
> unchanged name without explanation... But I think you're right though
> I was looking at it the other way around. I want to have an API for a
> two-stage sync and of course if I do that I'll comment it to explain
> that clearly.
>
> The gist of the comments was that the function is preparing to fsync
> to initiate the i/o early and allow the later fsync to fast -- but
> also at the same time have the beneficial side-effect of avoiding
> cache poisoning. It's not clear that the two are necessarily linked
> though. Perhaps we need two separate apis, though it'll be hard to
> keep them separate on all platforms.

Well, maybe we should start with a discussion of what kernel calls
you're aware of on different platforms and then we could try to put an
API around it.  I mean, right now all you've got is
POSIX_FADV_DONTNEED, so given just that I feel like the API could
simply be pg_dontneed() or something.  It's hard to design a general
framework based on one example.

...Robert

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