Strange raid behaviour

[Chris Eborn]

On 12/19/05, Chris Eborn <chris.eborn at gmail.com> wrote:
> On 12/15/05, Gilboa Davara <gilboada at netvision.net.il> 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 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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> >
>
>
 Hi Gilboa,

 [root at nucoda root]# blockdev --getra /dev/md0
 2147483647
 [root at nucoda root]# raidstart --all
 [root at nucoda root]# mount /dev/md0 /array
 [root at nucoda root]# blockdev --getra /dev/md0
 1024
 [root at nucoda root]#

 Also:

 [root at nucoda root]# blockdev  --setra 4096  /dev/md0
 BLKRASET: Invalid argument
 [root at nucoda root]# blockdev  --setra 256  /dev/md0
 BLKRASET: Invalid argument
 [root at nucoda root]# blockdev  --setra 255  /dev/md0
 [root at nucoda root]#


 Interesting! I was away on Friday, just back now. I will investigate
 this immeadiately.


 Thanks for the tip.

 Chris