Strange raid behaviour

[Gilboa Davara]

On Thu, 2005-12-15 at 17:07 +0000, Chris Eborn wrote:
> Does the output from the following  commands seem odd to people?
> 
> [root at nucoda root]# hdparm -t /dev/md0
> 
> /dev/md0:
>  Timing buffered disk reads:  774 MB in  3.01 seconds = 257.52 MB/sec
> [root at nucoda root]# mount /dev/md0 /array
> [root at nucoda root]# hdparm -t /dev/md0
> 
> /dev/md0:
>  Timing buffered disk reads:  416 MB in  3.00 seconds = 138.55 MB/sec
> [root at nucoda root]# umount /array
> [root at nucoda root]# hdparm -t /dev/md0
> 
> /dev/md0:
>  Timing buffered disk reads:  776 MB in  3.01 seconds = 257.42 MB/sec
> [root at 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
> 

I'm seeing the same behavior on my 4x36GB MD5 setup.
Mounted FS: ~105-110MB/s.
Unmount FS: ~125MB/s.

Can you check the read-ahead settings (blockdev --getra /dev/md0)
before /and/ after fs mount?

Gilboa