Subject: Re: Low AAC performance but only when tested through the file system
To: Olaf Seibert <email@example.com>
From: Jason Thorpe <firstname.lastname@example.org>
Date: 12/01/2003 22:13:11
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On Dec 1, 2003, at 8:41 AM, Olaf Seibert wrote:
> I have installed a FreeBSD test installation on part of the disk, and
> seems to suffer the same fate as NetBSD. In systat iostat it also does
> not get more than about 60 write xfers/second, and an associated
> transfer speed of 4M/sec. So this tends to point to hardware again,
> since FreeBSD's AAC driver seems much further developed than NetBSD's.
> How that RawIO benchmark managed to get its much better results is
> a mystery to me.
What's probably happening is that the RawIO benchmark performs I/O in
such a way that it is able to make good use of the card's on-board
stripe cache. Presumably RawIO is doing large sequential writes, which
are broken up into 64k chunks, but possibly in a way that the card is
able to optimize (i.e. hold other stripe chunks in its stripe cache
until the entire stripe is present and eliminate the r/m/w cycle on the
On the other hand, the file system is issuing 64k I/Os, but not
necessarily all of them adjacent, so the card is probably having a
harder time optimizing the I/O with the stripe cache.
You could probably get some improvement by tweaking your stripe depth
on the card. You really want to be writing an entire stripe at a time,
so setting your stripe depth to (64k / ndisks-1) for RAID-5 should do
the trick (64k because that is the maximum I/O size the NetBSD kernel
will currently issue).
NetBSD really needs to do some work tuning the file system to play nice
with RAID. In particular, tweaking the file system layout for the RAID
layout, and making sure to issue I/O that is stripe-aligned and
multiple-of-stripe-size would really improve the performance of the
file system on RAID-4 and RAID-5.
-- Jason R. Thorpe <email@example.com>
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