Date: Fri, 28 Apr 2006 14:30:36 +1200
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Hardware: HP StorageWorks MSA 1500
In-reply-to: <Pine.LNX.4.58.0604251642580.1140@sphinx.mythic-beasts.com>
To: Alex Hayward <xelah-pgsql@xelah.com>
Cc: Pgsql performance <pgsql-performance@postgresql.org>
Message-id: <44517E4C.3090809@paradise.net.nz>
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Alex Hayward wrote:

> 
> IO bound doesn't imply IO bandwidth bound. 14 disks doing a 1ms seek
> followed by an 8k read over and over again is a bit over 100MB/s. Adding
> in write activity would make a difference, too, since it'd have to go to
> at least two disks. There are presumably hot spares, too.
> 

Very true - if your workload is primarily random, ~100Mb/s may be enough 
bandwidth.

> I still wouldn't really want to be limited to 200MB/s if I expected to use
> a full set of 14 disks for active database data where utmost performance
> really matters and where there may be some sequential scans going on,
> though.
> 

Yeah - thats the rub, Data mining, bulk loads, batch updates, backups 
(restores....) often use significant bandwidth.

> Though, of course, these won't do many of the things you can do with a SAN
> - like connect several computers, or split a single array in to two pieces
> and have two computers access them as if they were separate drives, or
> remotely shut down one database machine and then start up another using
> the same disks and data. The number of IO operations per second they can
> do is likely to be important, too...possibly more important.
>

SAN flexibility is nice (when it works as advertised), the cost and 
performance however, are the main detractors. On that note I don't 
recall IO/s being anything special on most SAN gear I've seen (this 
could have changed for later products I guess).

> There's 4GB FC, and so presumably 4GB SANs, but that's still not vast
> bandwidth. Using multiple FC ports is the other obvious way to do it with
> a SAN. I haven't looked, but I suspect you'll need quite a budget to get
> that...
> 

Yes - the last place I worked were looking at doing this ('multiple 
attachment' was the buzz word I think) - I recall it needed special 
(read extra expensive) switches and particular cards...

Cheers

Mark