Subject: Re: Low AAC performance but only when tested through the file system
To: Olaf Seibert <>
From: Jason Thorpe <>
List: current-users
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 
> it
> 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 
> still
> 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 <>

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