random i/o speedup weirdness

Hi,

I'm running fio 1.16 directly on a disk partition for an IOPS test.
More specifically, I've created three partitions at the beginning, middle
and end of the disk to verify some claims that the "beginning" of the
disk is "faster", and to compare two disk types with regards to IOPS.

So here I am running random read I/O agains a 4GB partition and,
towards the end, the test speeds up considerably. This is true for all
three partitions on two different direct-attached single disks.

For example, running this:
--- cut here ---
[global]
bs=4k
direct=1
rw=randread
ioengine=libaio
iodepth=2
randrepeat=1
write_bw_log
write_lat_log

[/dev/sdb1]
--- cut here ---
produces the attached throughput plot. Could someone please explain why
it speeds up towards the end so significantly?
(iostat actually confirms what fio reports)

This is a recompiled RHEL4.4 kernel (2.6.9+).

Thanks a lot,

Andras

ps. btw all three partitions showed the same speed, and the same
"speedup", on both disks.