Re: [RFC] Process requests instead of bios to use a scheduler

[NeilBrown <neilb@suse.de>]

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On Mon, 02 Jun 2014 11:51:52 +0200 Sebastian Parschauer
<sebastian.riemer@profitbricks.com> wrote:


> > Having a scheduler for RAID0 doesn't make any sense to me.
> > RAID0 simply passes each request down to the appropriate underlying device.
> > That device then does its own scheduling.
> > 
> > Adding a scheduler may well make sense for RAID1 (the current "scheduler"
> > only does some read balancing and is rather simplistic) and for RAID4/5/6/10.
> > 
> > But not for RAID0 .... was that a typo?
> 
> Nope, we have our RAID-1+0. So it is more or less a RAID-10 and putting
> the scheduler to this RAID-0 layer makes sense for us.

I still cannot imagine how this would work.  RAID-0 has no decisions to make,
so no where for a scheduler to fit.

Just to clarify: is this md/raid0 over md/raid1 or md/raid0 over
hardware/raid1?


> > Could you do a graph?  I like graphs :-)
> > I can certainly seem something has changed here...
> 
> Sure, please find the graphs attached. I've converted it into percentage
> so that number of bios can be compared to number of requests.

Thanks.

> > 
> > Show me the code and I might be able to provide a more detailed opinion.
> 
> I would say let the user decide whether an MD device should be equipped
> with a scheduler or not. We can port our code to latest kernel + latest
> mdadm and send you a patch set for testing. Just give me some time to do it.

In the first instance, I just want to get a concrete idea of what you have
done because what you have said doesn't make sense to me.  I'm happy to look
at code against a not-quite-current kernel to get that idea.  But I'm also
happy for it to be against the latest, whatever suits you.

NeilBrown

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