Excuse me, but i original wrote the following E-Mail to Hyrum K. Wright
because I wasn't used to the guidelines of the subversion project.
----- Weitergeleitet von Michael Felke/AN/Stockhausen/DE am 24.06.2010
Thema: subversion Issue 2286: rep-sharing cache for fsfs
Hello Hyrum K. Wright,
sorry that i bother you with this directly, but i have no clue of work
with the issue tracker.
I just started to checking the changes in 1.6 on possible problem, when
updating our raw data repository to this version.
I found that the new representation caching would have an great impact on
It could save us a lot of disk space on the server, because the software
we are using, often generates file copies, witch are added as separate
But unfortunately it seems we could not use it :-(
Because after what the source code of rep.cache.c and fs_fs.c in
libsvn_fs_fs looks to me, the mechanism to find an already existing
representation is only relaying on the sha1-checksum.
Due to the possibility of hash collisions it's not enough to ensure that
the found old representation is really an duplicate of the new one.
An undetected hash collision would result in a file with a totally wrong
sha1 has been developed to detected modifications in a file and ensure
that it's likely impossible to generate the same sha1-checksum be only
modifying a file.
So it is good to use it whether a file has been modified.
But it's not designed to check if two different files could possibly the
There are always infinity numbers of independent files generating the same
Indeed, the number of hash collisions is only finite for a given file
size, but is still increasing dramatically with the file size.
So additional checking of the file size helps but is not a completely
The number of undetected hash collisions could be reduced easily by also
checking the md5-checksum, the size and the expanded-size.
To make this feature totally reliable, a complete comparison of the files
content with the content of the old representation found, is necessary
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Received on 2010-06-24 13:06:17 CEST