something is dramatically wrong with buffered read performance in 2.6.32.9

[Stan Hoeppner <stan@hardwarefreak.com>]

I recently upgraded from kernel 2.6.31.1 to 2.6.32.9.  Upon kicking the
tires on the new kernel I discovered that my two data partitions are
suffering a ~30MB/s performance loss compared to my root partition.  That's
a loss of nearly half the performance of my disk.

The root partition is a primary, formatted with EXT2.  The two data
partitions are logicals within an extended partition, both formatted with
XFS.  hdparm O_DIRECT tests show identical throughput for all 3 partitions.
 Going through the buffer cache, however, shows a huge performance drop for
the two logical partitions.  Throughput is nearly cut in half through the
buffer cache:

/dev/sda2:
 Timing O_DIRECT disk reads:  238 MB in  3.02 seconds =  78.79 MB/sec
/dev/sda2:
 Timing buffered disk reads:  208 MB in  3.00 seconds =  69.26 MB/sec

/dev/sda6:
 Timing O_DIRECT disk reads:  236 MB in  3.02 seconds =  78.14 MB/sec
/dev/sda6:
 Timing buffered disk reads:  126 MB in  3.01 seconds =  41.79 MB/sec

/dev/sda7:
 Timing O_DIRECT disk reads:  238 MB in  3.00 seconds =  79.27 MB/sec
/dev/sda7:
 Timing buffered disk reads:  126 MB in  3.03 seconds =  41.65 MB/sec

Any ideas what I'm running into here?  Did I somehow fubar something in
menuconfig?

I get the same results with dd copy tests through the buffer cache to
/dev/null.  Any idea what's wrong?  Is this a known bug?

-- 
Stan