Re: Linux MD RAID 1 read performance tunning

["Keld Jørn Simonsen" <keld@keldix.com>]

On Tue, Dec 22, 2009 at 07:08:25PM +0200, Ciprian Dorin, Craciun wrote:
> 2009/12/22 Keld Jørn Simonsen <keld@keldix.com>:
> > On Tue, Dec 22, 2009 at 06:34:55PM +0200, Ciprian Dorin, Craciun wrote:
> >>     Hello all!
> >>
> >>     I've created a 64G RAID 1 matrix from 3 real disks. (I intend to
> >> use this as a target for backups.)
> >>     Now while playing around with this array, I've observed that the
> >> read performance is quite low because it always reads from the disk in
> >> the first slot (which happens to be the slowest...)
> >>
> >>     So my questions are:
> >>     * is there any way to tell the MD driver to load-balance the reads
> >> between the three disks?
> >
> > It does not make sense to do distributed reading in raid1 for sequential
> > files. This is because it will not be faster to read from more drives,
> > as this will only make the reading from one drive skipping blocks on
> > that drive. In other words, in the time you use for skipping blocks on
> > one drive, you could just as well have read the blocks. So then better
> > just read all the blocks off one drive, and then do other possible IO
> > from other drives.
> 
>     Aha. It makes sens now. But, does it mean that if I have parallel
> IO's (from different read operations) they are going to be distributed
> between the disks?

It should, but I am not fully sure it does.
But try it out with two concurrent reads of two big files, and then
watch it with iostat 

Best regards
keld
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