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Re: SAN performance mystery

From: "Mikael Carneholm" <Mikael(dot)Carneholm(at)WirelessCar(dot)com>
To: "Tim Allen" <tim(at)proximity(dot)com(dot)au>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: SAN performance mystery
Date: 2006-06-16 11:45:09
Message-ID: 7F10D26ECFA1FB458B89C5B4B0D72C2B45ABFC@sesrv12.wirelesscar.com (view raw or flat)
Thread:
Lists: pgsql-performance
We've seen similar results with our EMC CX200 (fully equipped) when
compared to a single (1) SCSI disk machine. For sequential reads/writes
(import, export, updates on 5-10 30M+ row tables), performance is
downright awful. A big DB update took 5-6h in pre-prod (single SCSI),
and 10-14?h (don't recall the exact details) in production (EMC SAN).
And this was with a proprietary DB, btw - no fsync on/off affecting the
results here.

FC isn't exactly known for great bandwidth, iirc a 2Gbit FC channel tops
at 192Mb/s. So, especially if you mostly have DW/BI type of workloads,
go for DAD (Direct Attached Disks) instead.

/Mikael

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Tim Allen
Sent: den 15 juni 2006 23:50
To: pgsql-performance(at)postgresql(dot)org
Subject: [PERFORM] SAN performance mystery

We have a customer who are having performance problems. They have a
large (36G+) postgres 8.1.3 database installed on an 8-way opteron with
8G RAM, attached to an EMC SAN via fibre-channel (I don't have details
of the EMC SAN model, or the type of fibre-channel card at the moment). 
They're running RedHat ES3 (which means a 2.4.something Linux kernel).

They are unhappy about their query performance. We've been doing various
things to try to work out what we can do. One thing that has been
apparent is that autovacuum has not been able to keep the database
sufficiently tamed. A pg_dump/pg_restore cycle reduced the total
database size from 81G to 36G. Performing the restore took about 23
hours.

We tried restoring the pg_dump output to one of our machines, a
dual-core pentium D with a single SATA disk, no raid, I forget how much
RAM but definitely much less than 8G. The restore took five hours. So it
would seem that our machine, which on paper should be far less
impressive than the customer's box, does more than four times the I/O
performance.

To simplify greatly - single local SATA disk beats EMC SAN by factor of
four.

Is that expected performance, anyone? It doesn't sound right to me. Does
anyone have any clues about what might be going on? Buggy kernel
drivers? Buggy kernel, come to think of it? Does a SAN just not provide
adequate performance for a large database?

I'd be grateful for any clues anyone can offer,

Tim




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