Re: XFS buffered sequential read performance low after kernel upgrade

[Dave Chinner <david@fromorbit.com>]

On Fri, Mar 12, 2010 at 05:48:16AM -0600, Stan Hoeppner wrote:
> Hello,
> 
> I'm uncertain whether this is the best place to bring this up.  I've been
> lurking a short while and it seems almost all posts here deal with dev
> issues.  On the off chance this is an appropriate forum, here goes.
....
> hdparm test results for the filesystems:
> 
> /dev/sda2:
>  Timing O_DIRECT disk reads:  236 MB in  3.01 seconds =  78.48 MB/sec
> /dev/sda2:
>  Timing buffered disk reads:  172 MB in  3.03 seconds =  56.68 MB/sec
> 
> /dev/sda6:
>  Timing O_DIRECT disk reads:  238 MB in  3.00 seconds =  79.21 MB/sec
> /dev/sda6:
>  Timing buffered disk reads:  116 MB in  3.03 seconds =  38.27 MB/sec
> 
> /dev/sda7:
>  Timing O_DIRECT disk reads:  238 MB in  3.01 seconds =  79.10 MB/sec
> /dev/sda7:
>  Timing buffered disk reads:  114 MB in  3.00 seconds =  37.99 MB/sec

Those tests don't go through the filesystem - they go directly to the
block device. Hence if these are different to your old kernel, the
problem is outside XFS. I'd suggest the most likely cause is the
elevator - if you are using CFQ try turning off the new low-latency
mode, or changing to deadline or noop and see if the problem goes
away.

Also, the XFS partitions are on the inner edge of the disk, so they
are always going to be slower (can be as low as half the speed) than
partitions on the outer edge of the disk for sequential access...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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