Strange raid behaviour
Gilboa Davara
gilboada at netvision.net.il
Thu Dec 15 18:10:18 UTC 2005
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
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