From: Greg Smith <gsmith@gregsmith.com>
To: pgsql-performance@postgresql.org
Subject: Re: SAN vs Internal Disks
Date: Fri, 7 Sep 2007 16:04:41 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0709071542280.25508@westnet.com> (raw)
In-Reply-To: <20070907103341.GC20896@oppetid.no>
References: <a199704d0709060535q6fea139aq2e3863e685555b0d@mail.gmail.com>
<dcc563d10709060925o6f11f632sbfbc2b64530ae80d@mail.gmail.com>
<a199704d0709061003u71d536fbh9d71ed095dfe16b@mail.gmail.com>
<Pine.GSO.4.64.0709062354040.28915@westnet.com>
<20070907100446.GE1795@mathom.us>
<20070907103341.GC20896@oppetid.no>
On Fri, 7 Sep 2007, Tobias Brox wrote:
> We're also considering to install postgres on SAN - that is, my boss is
> convinced this is the right way to go.
> Advantages:
> 1. Higher I/O (at least the salesman claims so)
Shockingly, the salesman is probably lying to you. The very concept of
SAN says that you're putting something in between your system and the
disks, and that something therefore must slow things down compared to
connecting directly. iSCSI, FC, whatever you're using as the
communications channel can't be as fast as a controller card with a good
interface straight into the motherboard. For example, a PCI-E x16 disk
controller card maxes out at 4GB/s in each direction; good luck bonding
enough iSCSI or FC channels together to reach that transfer rate and
getting something even remotely cost-competative with an internal card.
The cases where a SAN can improve upon performance over direct discs are
when the comparison isn't quite fair; for example:
1) The SAN allows spreading the load over more disks than you can fit
internally in the system
2) The SAN provides a larger memory cache than the internal cards you're
comparing against
If you're in one of those situations, then perhaps the salesman's claim
could have some merit. There are lots of reasons one might want to use a
SAN, but a higher I/O rate when fairly comparing to connecting disks
directly is unlikely to be on that list.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: gsmith@gregsmith.com
Subject: Re: SAN vs Internal Disks
In-Reply-To: <Pine.GSO.4.64.0709071542280.25508@westnet.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox