Re: Adding more drives/saturating the bandwidth

[Robin Hill <robin@robinhill.me.uk>]

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On Fri Apr 03, 2009 at 10:42:20PM +0200, Goswin von Brederlow wrote:

> Richard Scobie <richard@sauce.co.nz> writes:
> 
> > Goswin von Brederlow wrote:
> >
> >>
> >> Now think about the same with 6 disk raid5. Suddenly you have partial
> >> stripes. And the alignment on stripe boundaries is gone too. So now
> >> you need to read 384k (I think) of data, compute or delta (whichever
> >> requires less reads) the parity and write back 384k in 4 out of 6
> >> cases and read 64k and write back 320k otherwise. So on average you
> >> read 277.33k and write 362.66k (= 640k combined). That is twice the
> >> previous bandwidth not to mention the delay for reading.
> >>
> >> So by adding a drive your throughput is suddenly halfed. Reading in
> >> degraded mode suffers a slowdown too. CPU goes up too.
> >>
> >>
> >> The performance of a raid is so much dependent on its access pattern
> >> that imho one can not talk about a general case. But note that the
> >> more drives you have the bigger a stripe becomes and you need larger
> >> sequential writes to avoid reads.
> >
> > I take your point, but don't filesystems like XFS and ext4 play nice
> > in this scenario by combining multiple sub-stripe writes into stripe
> > sized writes out to disk?
> >
> > Regards,
> >
> > Richard
> 
> Some FS have a parameter to tune to the stripe size. If that actually
> helps or not I leave for you to test.
> 
> But ask yourself: Have any a tool to retune after you've grown the raid?
> 
Both XFS and ext2/3 (and presumably 4 as well) allow you to alter the
stripe size after growing the raid (ext2/3 via tune2fs and XFS via mount
options).  No idea about other filesystems though.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
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