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Slow read performance

To: linux-xfs@xxxxxxxxxxx
Subject: Slow read performance
From: Jason Howard <jason@xxxxxxxxxxxxx>
Date: Fri, 20 Aug 2004 13:42:53 -0700
Organization: SpectSoft, LLC
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Hello All,

I am wondering if anyone can offer any insight into a problem I have been 
seeing with XFS included in the 2.4.27 kernel.  I am seeing a huge difference 
between a raw device (/dev/md0) read and a filesystem read when reading from 
an array that is capable of >500 MBytes/sec.  The reads and writes are 
sequential, both on the raw disk device and XFS filesystem (using sequential 
files).

Here is an example of what we normally see on a fairly slow array:
RAW PERFORMANCE
Read Average : 212.674 MB/SEC over 1000 frames
Write Average: 234.769 MB/SEC over 1000 frames
FS PERFORMANCE
Read Average : 148.125 MB/SEC over 1000 frames
Write Average: 286.031 MB/SEC over 1000 frames

Now here is the problem I am seeing on a fast array:
RAW PERFORMANCE
Read Average : 594.965 MB/SEC over 1000 frames
Write Average: 350.604 MB/SEC over 1000 frames
FS PERFORMANCE
Read Average : 67.280 MB/SEC over 1000 frames
Write Average: 305.363 MB/SEC over 1000 frames

I have seen similar results on two separate arrays, one a software raid of 
dual U320 arrays, the other is a software raid of dual 2G fibre w/ 16 disks.  
I don't suspect it is a problem with the physical interfaces, as I am seeing 
the same issue on multiple technologies.  Also, I have pretty much ruled out 
a problem with Linux software raid as we are getting expected performance 
from the raw /dev/md0 device.  That is not to say that it couldn't very well 
be a nasty interaction between software raid and XFS.

Thanks much!
Jason

- -- 
 Jason Howard
 
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   SpectSoft, LLC
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