From: Joost Kraaijeveld <J.Kraaijeveld@Askesis.nl>
To: David Lang <david@lang.hm>
Cc: Richard Huxton <dev@archonet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: x206-x225
Date: Sat, 11 Mar 2006 09:17:09 +0100
Message-ID: <1142065029.6634.23.camel@localhost> (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, 2006-03-10 at 23:57 -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.
Not according to a conversation I had with Western Digital about the
write performance of my own SATA disks. What I understand from their
explanation their disk are limited by the MB/sec and not by the number
of writes/second, e.g. I could write 50 MB/sec *in 1 bit/write* on my
disk. This would suggest that the maximum transactions of my disk
(overhead of OS and PostgreSQL ignored) would be 50MB / (transaction
size in MB) per second. Or am I missing something (what would not
surprise me, as I do not understand the perforance of my system at
all ;-))?
--
Groeten,
Joost Kraaijeveld
Askesis B.V.
Molukkenstraat 14
6524NB Nijmegen
tel: 024-3888063 / 06-51855277
fax: 024-3608416
e-mail: J.Kraaijeveld@Askesis.nl
web: www.askesis.nl
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To: pgsql-performance@postgresql.org
Cc: J.Kraaijeveld@Askesis.nl, david@lang.hm, dev@archonet.com
Subject: Re: x206-x225
In-Reply-To: <1142065029.6634.23.camel@localhost>
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