[sheepdog] [PATCH 0/7] modify hash calculation

[MORITA Kazutaka]

At Mon, 2 Sep 2013 10:56:11 +0800,
Liu Yuan wrote:
> 
> On Fri, Aug 30, 2013 at 06:32:02PM +0900, MORITA Kazutaka wrote:
> > From: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> > 
> > The current hash algorithm is fast but shows poor dispersion.  This
> > series introduces a new hash function based on the current fnv1a
> > algorithm.
> > 
> > I compared performance and dispersion when generating 64 bit integer
> > values with:
> > 
> >  - hash_64 (a simple hash function used in Linux kernel)
> >  - fnv1a   (the current hash function)
> >  - sd_hash (the one this patchset introduces)
> >  - sha1
> > 
> > and sd_hash showed a good result.
> > 
> >                    hash_64    fnv1a   sd_hash     sha1
> > Performance (*1)     43 ms    96 ms    182 ms   2387 ms
> > Dispersion (*2)     9216.0   2927.7      11.4      6.57
> > 
> > (*1) The time to generate 10,000,000 hash values.
> > (*2) The result of chi-squared test.  It should be less than 16.9.
> > 
> > Please note that this series breaks backward compatibility, and
> > shouldn't be merged into the stable trees.
> 
> Does this means that we can't upgrade old cluster to master after this series
> is merged (people should format the cluster)?

What I mean is that such a fundamental change shouldn't go into the
stable trees.

I think the object reclaim feature will be also merged before the v0.8
release and it will change the format too.  I'm going to write an
upgrade code after the v0.8 format is fixed.

Thanks,

Kazutaka