To: "Alex Turner" <armtuk@gmail.com>
Cc: "Mark Lewis" <mark.lewis@mir3.com>,  "Tom Lane" <tgl@sss.pgh.pa.us>,
	"Brian Hurt" <bhurt@janestcapital.com>, "Tim Allen" <tim@proximity.com.au>,
	pgsql-performance@postgresql.org
Subject: Re: SAN performance mystery
References: <4491D61B.9040507@proximity.com.au>
	<4491D8DC.4070906@janestcapital.com> <1751.1150410284@sss.pgh.pa.us>
	<1150413917.31200.109.camel@archimedes>
	<33c6269f0606151658h731915d3k405b4da67c49228a@mail.gmail.com>
In-Reply-To: <33c6269f0606151658h731915d3k405b4da67c49228a@mail.gmail.com>
From: Greg Stark <gsstark@mit.edu>
Organization: The Emacs Conspiracy; member since 1992
Date: 16 Jun 2006 07:28:35 -0400
Message-ID: <87psh98hlo.fsf@stark.xeocode.com>
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"Alex Turner" <armtuk@gmail.com> writes:

> Given the fact that most SATA drives have only an 8MB cache, and your RAID
> controller should have at least 64MB, I would argue that the system with the
> RAID controller should always be faster.  If it's not, you're getting
> short-changed somewhere, which is typical on linux, because the drivers just
> aren't there for a great many controllers that are out there.

Alternatively Linux is using the 1-4 gigabytes of cache available to it
effectively enough that the 64 megabytes of mostly duplicated cache just isn't
especially helpful...

I never understood why disk caches on the order of megabytes are exciting. Why
should disk manufacturers be any better about cache management than OS
authors?

In the case of RAID 5 this could actually work against you since the RAID
controller can _only_ use its cache to find parity blocks when writing.
Software raid can use all of the OS's disk cache to that end.

-- 
greg