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Re: BDB vs FSFS - OMG!

From: Stefan Fuhrmann <stefan.fuhrmann_at_wandisco.com>
Date: Sun, 6 Jan 2013 15:51:16 +0100

On Sun, Jan 6, 2013 at 3:10 PM, Daniel Shahaf <danielsh_at_elego.de> wrote:

> Branko Čibej wrote on Sun, Jan 06, 2013 at 11:27:09 +0100:
> > On 06.01.2013 10:43, Bert Huijben wrote:
> > > The revprop and revision cache are in fsfs, not the repos layer...
> > >
> > > In what way are you then comparing the backends?
> > >
> > > You are now comparing a backend+caching to a backend without caching.

I used "backend" as a term for FS API implementation.

> > I’m not against dropping support, but if we do it we should do it for
> > > the right reasons, not by using skewed numbers.

As others already pointed out, I don't think those numbers
are skewed. They answer a simple question: What are the
size & performance differences between both implementations
in a typical setup? IOW, what a user would gain when switching
from BDB to FSFS considering that such a conversion is not
for free.

The claim here is not that BDB itself was rubbish but that our
current use of it results in a much worse user experience
than FSFS.

> Caching is part of the FSFS backend. One would assume that a key-value
> > database like BDB would have its own cache, which is therefore
> > implicitly part of the BDB back-end. I don't see how you could construe
> > these numbers as skewed.
> Just look at all the different svn_cache__t instances in FSFS. They
> include, for example, a cache of combined svndiff windows --- that's not
> something the db layer is going to have.

While that is true, there plenty of algorithmic improvements
have been made to the FSFS code as well. I just re-ran one
of the benchmarks with a completely default svnserve
configuration (16M cache, no fulltext, txdelta nor revprop
caching). Results:

(1): (2) : (3)
 1 : 6.8 : 5.0 svn-bench null-log ^/trunk -v -g
 1 : 15 : 5.8 (second run)

The missing delta and revprop caches hurt the space-optimized
FSFS configuration (3) much more than FSFS defaults.

-- Stefan^2.

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