Hi Jeff
Yes, it is very important because the disks will see random reads
(many clients competing will make it random, even if each client reads
sequentially). That is why increasing data block size helps so much.
I'm told here at slac that the seek time is divorced from density, and
it is mostly dominated by the fact that heads have to move at all, the
distance how far they have to move is not so important. So there is very
little to plot, on average it is going to be around 6 msec (until the
technology completely changes)
Jacek
Jeffrey P Kantor wrote:
Hi Jacek,
If the seek time is that significant, perhaps we should model this as a
trend over time rather than a point.
Jeff
From: Jacek Becla <[EMAIL PROTECTED]>
Organization: Stanford Linear Accelerator Center
Reply-To: LSST Data Management <[email protected]>
Date: Thu, 06 Jul 2006 13:09:36 -0700
To: LSST Data Management <[email protected]>
Cc: Don Dossa <[EMAIL PROTECTED]>
Subject: [LSST-data] disk seek time?
Hello,
One important knob that can significantly change database
disk IO is disk seek time. The disk model sent by Jim Gray
suggested 4 msec seek time and transfer rate 250 MB/sec, and
that is what I was using for estimating disk IO so far.
Our local (SLAC) hardware experts tell me 4 msec is optimistic,
and 6 msec would probably be more realistic (and it is unlikely
to change any time soon). The transfer rate on the other hand
is likely to go up because of the density growth. Unfortunately
this does not seem to help much for the page sizes we consider
(~128-256KB). The seek time change dominates, e.g. if I change
seek time from 4 to 6 msec and triple the transfer rate,
required number of disks increases almost proportionally to
seek time increase (e.g. from from 6K to 9K).
So I think I will need a bit of guidance from TechAssessWG regarding
what disk seek time I should assume for LSST for the year 2013.
For convenience I'm attaching the disk model provided by Jim.
thanks,
Jacek
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