disk performance strange/low

[Christoph Hellwig]

On Mon, Aug 17, 2009 at 11:11:07PM +0200, Michael Monnerie wrote:
> I've got an Areca RAID Controller with 5x 1TB disks, and in the last 
> time I often found it to be strange. Now I investigated, and found this. 
> >From another server I NFS mount a share, and make
> dd if=/dev/sda of=/server/share/test.dd bs=1024k
> so that should do big sequential writes. But the performance goes up and 
> down, like you see in this output from "iostat -kx 5 555". You see write 
> KB/s is 5824, 23207, 87283, 704, 40187. Up and down all the time. Could 
> that be solved by the patch you discussed last time, or is there 
> something else?

Yes, that sounds like the problem of the VM not feeding us enough
pages to write.  What kernel is this on?  Latest 2.6.31-rc has a
workaround for it.