2.6.29.1 vs 2.6.27.21 ext4 performance problem

["Alexander V. Lukyanov" <lav@yar.ru>]

I have a server that is busy reading and writing several disks with ext4 fs.
After booting 2.6.29.1 kernel I have noticed significant increase in read rate.
I measure the read rate using a script which reads sysfs byte and access counts
and calculates average per second.

Here is the output of the script from 2.6.29.1 kernel:

sda: read-rate=58932   (r=4163)
sdb: read-rate=2570440 (r=40763)
sdc: read-rate=3320403 (r=52756)
sdd: read-rate=3199639 (r=52885)
SUM: read-rate=9149414 (r=150567)

And this is output from 2.6.27.21:

sda: read-rate=32768   (r=4096)
sdb: read-rate=204800  (r=24576)
sdc: read-rate=204800  (r=22528)
sdd: read-rate=704512  (r=45056)
SUM: read-rate=1146880 (r=96256)

The rate is in bytes per second, "r" is accesses per second.
One can notice that the access rate is similar (the work load on the server is
basically the same), but byte rate is much higher.

I don't yet know what causes this reading increase. Any ideas?

-- 
   Alexander.