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Date: Sat, 29 Mar 2008 10:34:07 +0000
From: James Mansion <james@mansionfamily.plus.com>
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Subject: Re: Planning a new server - help needed
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Greg Smith wrote:
> As for SCSI vs. SATA, I collected up the usual arguments on both sides 
> at http://www.postgresqldocs.org/index.php/SCSI_vs._IDE/SATA_Disks
>
Why do you claim that 'More platters also means slower seeks
and generally slower performance.'?

On the face of it, it should mean that the number of track step
operations is reduced, even if the drive doesn't buffer a slice
of tracks aross all platters (which would help if it did).

I'm not entirely sure why the extra platters should really count
as more moving parts since I think the platter assembly and
head assembly are both single parts in effect, albeit they will
be more massive with more platters. I'm not sure how much
extra bearing friction that will mean, but it is reasonable that
some extra energy is going to be needed.

Recent figures I've seen suggest that the increased storage
density per platter, plus the extra number of platters, means
that the streaming speed of good 7200rpm SATA drives is
very close to that of good 15000rpm SAS drives - and you
can choose which bit of the disk to use to reduce seek time and
maximise transfer rate with the oversize drives. You can
get about 100MB/s out of both technologies, streaming.

It may be worth considering an alternative approach. I suspect
that a god RAID1 or RAID1+0 is worthwhile for WAL, but
you might consider a RAID1 of a pair of SSDs for data. They
will use a lot of your budget, but the seek time is negligible so the
effective usable performance is higher than you get with
spinning disks - so you might trade a fancy controller with
battery-backed cache for straight SSD.

I haven't done this, so YMMV.  But the prices are getting
interesting for OLTP where most disks are massively
oversized. The latest Samsung and SanDisk are expensive
in the UK but the Transcend 16GB TS16GSSD25S-S  SATA
is about $300 equiv - it can do 'only' 'up to' 28MB/s write and
you wouldn't want to put WAL on one, but sustaining
15-20MB/s through random access to a real disk isn't
trivial. If average access is 10ms, and you write 100MB/s
streaming, then you have to ask yourself if you going to do
80 or more seeks a second.

James

James