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
