Re: performance problems with raid10,f2

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Tue, Mar 25, 2008 at 01:22:03PM +0000, Peter Grandi wrote:
> >>>> I have a 4 drive array with 1 TB Hitachi disks, formatted
> >>>> as raid10,f2 I had some strange observations: 1. while
> >>>> resyncing I could get the raid to give me about 320 MB/s in
> >>>> sequential read, which was good. After resync had been
> >>>> done, and with all 4 drives active, I only get 115 MB/s.
> 
> [ ... ]
> 
> >> Is this with, or without, your patch to avoid "read-balancing"
> >> for raid10/far layouts?  It sounds like it is without that
> >> patch ????
> 
> > I tried both without the patch and with the patch, with almost
> > same resulte.
> 
> That could be the usual issue with apparent pauses in the stream
> of IO requests to the array component devices, with the usual
> workaround of trying 'blockdev --setra 65536 /dev/mdN' and see if
> sequential reads improve.

Yes, that did it! 

> > Is resync building some table, and could that be it?  Or could
> > it be some time of inode traffic?
> 
> One good way to see what is actually happening is to use either
> 'watch iostat -k 1 2' and look at the load on the individual MD
> array component devices, or use 'sysctl vm/block_dump=1' and look
> at the addresses being read or written.

Good advice. I added your info to the wiki.

best regards
keld