Re: Strange raid behaviour

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On 12/20/05, Christian Motta <chris@xxxxxxxxx> wrote:
>  After seeing many of these threads I compared what my server tested at
> (mounted)
>  and now I am concerned about my own results.
>
>  My raid is raid 1 w/ 2 300gb Western Digital SATA/7200/8 hard drives
>
>  # /sbin/hdparm -t /dev/md4
>
>  /dev/md4:
>   Timing buffered disk reads:  172 MB in  3.00 seconds =  57.32 MB/sec
>
>  so if I am reading this correctly my hard drives are half the speed of my
> nic card????
>
>  Is that correct?
>
>  -chris
>
>
>
>
>
>  Chris Eborn wrote:
>  On 12/15/05, Gilboa Davara <gilboada@xxxxxxxxxxxxxxxx> wrote:
>
>
>  On Thu, 2005-12-15 at 17:07 +0000, Chris Eborn wrote:
>
>
>  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
>
>
>  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
>
>
>  Hi There,
> Well I am still suffering with this - I have installed Fedora Core 2
> with kernel 2.6.14 and the raid is still going half speed.
> I have spent quite a while playing with the readahead values (blockdev
> --setra) on both the individual disks and the raid device (/dev/md0)
> and they do not seem to change anything, even with extreme values
> (like "blockdev --setra 0 /dev/sd[abcdefghi]"). Leads me to think that
> readahead has been disabled elsewhere.

Hi Christian,
It may well be worth testing with a proper utility like iozone or
bonnie as well.
Could you test the read speed of a single disk?

hdparm -t /dev/hda

My single disk will do around 60MB/sec (10,000rpm scsi disks:-)

Chris


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