Received: from localhost (unknown [200.46.204.191]) by postgresql.org (Postfix) with ESMTP id AF9249FB646 for ; Fri, 7 Sep 2007 17:04:47 -0300 (ADT) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.191]) (amavisd-maia, port 10024) with ESMTP id 39892-02 for ; Fri, 7 Sep 2007 17:04:42 -0300 (ADT) X-Greylist: from auto-whitelisted by SQLgrey-1.7.5 Received: from westnet.com (westnet.com [216.187.52.2]) by postgresql.org (Postfix) with ESMTP id C26CA9FB35D for ; Fri, 7 Sep 2007 17:04:42 -0300 (ADT) Received: from westnet.com (localhost [127.0.0.1]) by westnet.com (8.14.0/8.14.0) with ESMTP id l87K4fhW002809 for ; Fri, 7 Sep 2007 16:04:41 -0400 (EDT) Received: from localhost (gsmith@localhost) by westnet.com (8.14.0/8.13.2/Submit) with ESMTP id l87K4fJC002802 for ; Fri, 7 Sep 2007 16:04:41 -0400 (EDT) X-Authentication-Warning: westnet.com: gsmith owned process doing -bs Date: Fri, 7 Sep 2007 16:04:41 -0400 (EDT) From: Greg Smith X-X-Sender: gsmith@westnet.com To: pgsql-performance@postgresql.org Subject: Re: SAN vs Internal Disks In-Reply-To: <20070907103341.GC20896@oppetid.no> Message-ID: References: <20070907100446.GE1795@mathom.us> <20070907103341.GC20896@oppetid.no> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200709/136 X-Sequence-Number: 26681 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