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From Daniel Shahaf <...@daniel.shahaf.name>
Subject Re: Backward or forward deltas, backend, FSX
Date Fri, 16 Feb 2018 03:17:38 GMT
Nathan Hartman wrote on Thu, 15 Feb 2018 22:01 -0500:
> On Feb 15, 2018, at 4:11 PM, Daniel Shahaf <d.s@daniel.shahaf.name> wrote:
> > 
> > Péter wrote on Thu, 15 Feb 2018 19:47 +0100:
> > I'm not sure why you say "at least" 17 deltas.  The default value of
> > max-linear-deltification (see fsfs.conf) is 16, meaning that no fulltext
> > will require 17 delta applications to produce.
> > 
> 
> Does this mean that when checking out HEAD from a repository with a 
> large number of commits, the oldest revision it must access is HEAD - 
> 16?

No.

Besides, if that were the case, then any commit to the ASF repository, which
has over 200 projects, would have had to replicate the contents of 184 projects
in full (including all tags and branches).  That would be impractical and inefficient.

> In other words, is a compressed full text stored every 16 revisions?

No, for two reasons.

First, we're talking about revisions of a particular file here: not 'svn log -q ^/'
but 'svn log -q file.c'.

Second, Subversion uses skip-deltas, so a fulltext would be stored only
every O(2**16) ≈ O(65k) revisions (of a particular file):

https://svn.apache.org/repos/asf/subversion/trunk/notes/skip-deltas

Cheers,

Daniel

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