Strange raid behaviour

[Chris Eborn <chris eborn gmail com>]

Does the output from the following  commands seem odd to people?

[root nucoda root]# hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads:  774 MB in  3.01 seconds = 257.52 MB/sec
[root nucoda root]# mount /dev/md0 /array
[root nucoda root]# hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads:  416 MB in  3.00 seconds = 138.55 MB/sec
[root nucoda root]# umount /array
[root nucoda root]# hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads:  776 MB in  3.01 seconds = 257.42 MB/sec
[root nucoda root]#


Tha array is made of 10 scsi disks - each of which will read at around
60 MB/sec and all seems fine with the raid
device until the fs is mounted and it appears to halve the trasnfer
rate! There is no activity in the filesystem. At the moment this
is with FC2, though I have tried with FC3 and numerous kernels.
Tweaking around with block sizes and thing does change the speed of
the raw /dev/md0 -  but the speed always drops when I mount the
filesystem. Can anybody explain this behaviour?
Maybe I am missing something obvious, but this is baffling me - and
what is worse is that it all worked until I upgraded the system. I
have tried to go back, even put a new system disk in and started from
scratch, but cannot get a decent read speed from the array.

Chris