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re: performance over multiple disks

To: James Rich <james@xxxxxxxxxxxxx>, XFS mailing list <linux-xfs@xxxxxxxxxxx>
Subject: re: performance over multiple disks
From: Greg Freemyer <freemyer@xxxxxxxxxxxxxxxxx>
Date: Thu, 31 Oct 2002 17:30:58 -0500
Organization: The NorcrossGroup
Sender: linux-xfs-bounce@xxxxxxxxxxx
 >>  On another mailing list a debate arose about performance over a single
 >>  disk vs. multiple disks.  It goes something like this:

 >>  Suppose you have a 6 megabyte file stored on disk.  Would it be read
 >>  faster if it were stored contiguously on a single disk or spread over
 >>  multiple (say 4) disks?

 >>  It seems to me that as you get smaller it is faster for the single disk
 >>  case (remember that we are assuming the file is stored contiguously - not
 >>  spread all over the disk).  At some size it seems natural that it would be
 >>  faster if the file were spread over multiple disks.  Can anyone comment on
 >>  how XFS would perform?  I don't have the equipment available to test this,
 >>  but I'm not too concerned with actual benchmark numbers.  Mostly I'm just
 >>  wondering if I understand the filesystem correctly.

 >>  James Rich


I don't fully understand the logic, but I have a Compaq Storage Performance 
guide in front of me.

For high data rate applications such as yours it recommends a stripe width (SW) 
of 17 sectors.  It says that you want this small to get the spindles working in 
parallel, but if you go below 17, you start getting excessive overhead.

i.e. 17 sectors of contiguous data per drive.

For high request rate, it recommends the below stripe widths (SW):

highly localized requests: SW = 10x avg. transfer size

highly non-localized requests: SW = 20x avg. transfer size

unknown localization: SW = 15x avg. transfer size.

For all cases, they recommend your SW be a prime number.  So the above just get 
you in the neighborhood and you select the closest prime number.

Greg Freemyer
Internet Engineer
Deployment and Integration Specialist
Compaq ASE - Tru64 v4, v5
Compaq Master ASE - SAN Architect
The Norcross Group

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