Re: [PATCH v2 1/5] md/raid10: optimize read_balance() for 'far offset' arrays

[NeilBrown <neilb@suse.de>]

On Wed, 15 Jun 2011 23:35:53 +0900 Namhyung Kim <namhyung@gmail.com> wrote:

> Namhyung Kim <namhyung@gmail.com> writes:
> > 2011-06-15 (수), 08:51 +0200, Keld Jørn Simonsen:
> >> On Wed, Jun 15, 2011 at 11:02:00AM +0900, Namhyung Kim wrote:
> >> > If @conf->far_offset > 0, there is only 1 stripe so that we can treat
> >> > the array same as 'near' arrays.
> >> 
> >> does it also work with more than 2 copies - eg 3 copies?
> >> I think the original code just takes the available data blocks with the 
> >> lowest address.
> >> 
> >
> > Hi,
> >
> > Let me clarify this: AFAIK, 'far offset' array saves redundant data in
> > the diagonally adjacent chunk/disk, so it could be roughly thought as
> > 'raid0' array with reduced size - just ignore redundant chunks here. It
> > was my mistake considering it as 'near' array. :(
> >
> 
> I'm confused again. If fo > 0 && fc > 1 && nc > 1 then it turns out to
> a near array with reduced size, no? Does it still need to be treaded
> as RAID0?

This would be a mix of near and offset.  I'm not at all sure what the "best"
read balancing approach would be.  But as I don't think anyone would ever
actually use it, I don't think it really matters.

Thanks,
NeilBrown



> 
> 
> > Therefore, it makes more sense distributing reads over the array based
> > on some criteria - here, the address of starting sector - like RAID0
> > does. Now I see that the same goes to the 'far copies' array exactly, so
> > the original code is correct.
> >
> > Thanks.
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