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Re: fs-rep-sharing branch

From: Daniel Berlin <dberlin_at_dberlin.org>
Date: Thu, 23 Oct 2008 18:29:47 -0400

It's definitely not 2^128

Assuming a perfectly even distribution (which md5 doesn't have), the
birthday paradox means to expect a collision after 1.25 * sqrt(x)
outputs.

sqrt(2^128) = 2^64

Probabilistically, you have a 1% chance of collision after ~2^59
nodes, and it grows fairly quickly from there:
25% chance after 2^60, 50% chance after 2^61, etc

Again, this is all with a perfectly even distribution. If MD5's
distribution is say, "half as good as perfect", you will get
collisions with a little more than 4 billion files.

In any case, having a 64 bit number of files is getting within the
reach of large systems.
We should move to SHA1, which is in the "universe size number of files" range.

On Tue, Oct 21, 2008 at 9:50 PM, Greg Stein <gstein_at_gmail.com> wrote:
> There is a HUGE difference between constructing two files with the
> same md5 in order to falsify a signature, and that of two files in a
> repository having the same md5 hash by accident.
>
> Sit down and look at the odds. 1 in 2^128. If I understand my powers
> of two properly, I believe that means the earth is more likely to
> spontaneously explode, than for two files to have the same hash key.
>
> Cheers,
> -g
>
> On Tue, Oct 21, 2008 at 3:57 PM, David Glasser <glasser_at_davidglasser.net> wrote:
>> As far as I can tell from reading the source, this (at least in FSFS)
>> assumes that reps sharing the same md5 are the same file. (BDB seems
>> to use sha1.)
>>
>> This means that you cannot store two files with the same md5 in the
>> same repository. While obviously all hashes have collisions in
>> theory, md5 has collisions in practice: there are known instances.
>> And you know, cryptography researchers use Subversion! (At one point
>> I tried to help fix Ron Rivest's corrupted svn repo...) I do not
>> think that this limitation is appropriate for Subversion; I would
>> highly advise against releasing this without changing FSFS to use SHA
>> as well. (I can't find a mailing-list discussion of this choice; my
>> apologies if I missed one, I have admittedly been not paying as much
>> attention as I'd like to Subversion development recently.)
>>
>> --dave
>>
>> On Mon, Oct 6, 2008 at 8:59 PM, Hyrum K. Wright
>> <hyrum_wright_at_mail.utexas.edu> wrote:
>>> The fs-rep-sharing branch is functionally complete, and I'd like to get the
>>> branch merged to trunk soon. These are the stats for various copies of of our
>>> repository for the different branch/backend combinations.
>>>
>>> BDB: 1.5: 1.4GB
>>> trunk: 627MB
>>> reps-shared: 490MB
>>>
>>> FSFS: 1.5: 586MB
>>> trunk: 578MB
>>> reps-shared: 523MB
>>>
>>> The effect is quite pronounced on BDB, with around a 20% space savings compared
>>> with our current trunk (and over 67% compared with 1.5!) FSFS doesn't show as
>>> much improvement, partly due to the size of the index required to enable
>>> rep-sharing, partly due to decreased sharing opportunities in same-revision and
>>> parallel revision objects, and mostly due to the absolute floor on repo size due
>>> to inode usage.
>>>
>>> We may be able to tune the FSFS implementation just a bit. For instance, it may
>>> not be likely that directory content representations are likely to be shared, in
>>> which case we shouldn't bother
>>>
>>> The remaining issue is the failing blame tests. Blame tests 10 and 11, which
>>> test 'blame -g', both fail for both backends. Before the recent commits to add
>>> rep-sharing to fsfs, the tests only failed for bdb. I'm slightly puzzled here
>>> because 'blame -g' should be FS-agnostic. If anybody has some insight, I
>>> welcome it.
>>>
>>> [Note: Because SQLite is still not an official dependency, to compile the
>>> rep-sharing stuff with FSFS, you'll need to add -DENABLE_SQLITE_TESTING to the
>>> CPPFLAGS when configuring.]
>>>
>>> -Hyrum
>>>
>>>
>>>
>>
>>
>>
>> --
>> David Glasser | glasser@davidglasser.net | http://www.davidglasser.net/
>>
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Received on 2008-10-24 00:30:02 CEST

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