From: Jim C. Nasby <jnasby@pervasive.com>
To: David Lang <david@lang.hm>
Cc: Joost Kraaijeveld <J.Kraaijeveld@Askesis.nl>
Cc: Richard Huxton <dev@archonet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: x206-x225
Date: Tue, 14 Mar 2006 14:32:53 -0600
Message-ID: <20060314203253.GT45250@pervasive.com> (raw)
In-Reply-To: <Pine.LNX.4.62.0603102355550.23148@qnivq.ynat.uz>
References: <GMEAJIDMNDKCIHMMHDFLCEKHDHAA.hjs@rmax.nl>
<441181C6.3040401@archonet.com>
<1142063378.6634.13.camel@localhost>
<Pine.LNX.4.62.0603102355550.23148@qnivq.ynat.uz>
On Fri, Mar 10, 2006 at 11:57:16PM -0800, David Lang wrote:
> On Sat, 11 Mar 2006, Joost Kraaijeveld wrote:
>
> >On Fri, 2006-03-10 at 13:40 +0000, Richard Huxton wrote:
> >>Your ATA disk is lying about disk caching being turned off. Assuming
> >>each insert is in a separate transaction, then it's not going to do
> >>10,000 / 6 = 1667 transactions/sec - that's faster than it's rotational
> >>speed.
> >Could you explain the calculation? Why should the number of transactions
> >be related to the rotational speed of the disk, without saying anything
> >about the number of bytes per rotation?
>
> each transaction requires a sync to the disk, a sync requires a real
> write (which you then wait for), so you can only do one transaction per
> rotation.
But shouldn't it be possible to batch up WAL writes and syncs? In other
words, if you have 5 transactions that all COMMIT at exactly the same
time, it should be possible to get all 5 WAL pages (I'll assume each
one generated a small enough change so as not to require multiple WAL
pages) to the drive before the platter comes around to the right
position. The drive should then be able to write all 5 at once. At
least, theoretically...
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
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To: pgsql-performance@postgresql.org
Cc: jnasby@pervasive.com, david@lang.hm, J.Kraaijeveld@Askesis.nl, dev@archonet.com
Subject: Re: x206-x225
In-Reply-To: <20060314203253.GT45250@pervasive.com>
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