On Mon, Jan 08, 2007 at 01:37:45AM -0600, Evan Harris wrote:
> I've been playing with rsync and very large files approaching and 
> surpassing 100GB, and have found that rsync has excessively very poor 
> performance on these very large files, and the performance appears to 
> degrade the larger the file gets.

Yes, this is caused by the current hashing algorithm that the sender
uses to find matches for moved data.  The current hash table has a fixed
size of 65536 slots, and can get overloaded for really large files.

There is a diff in the patches dir that makes rsync work better with
large files: dynamic_hash.diff.  This makes the size of the hash table
depend on how many blocks there are in the transfer.  It does speed up
the transfer of large files significantly, but since it introduces a mod
(%) operation on a per-byte basis, it slows down the transfer of normal
sized files significantly.

I'm going to be checking into using a hash algorithm with a table that
is always a power of 2 in size as an alternative implementation of this
dynamic hash algorithm.  That will hopefully not bloat the CPU time for
normal-sized files.  Alternately, the hashing algorithm could be made to
vary depending on the file's size.  I'm hoping to have this improved in
the upcoming 3.0.0 release.

And one final thought that occurred to me:  it would also be possible
for the sender to segment a really large file into several chunks,
handling each one without overlap, all without the generator or the
receiver knowing that it was happening.  The upside is that huge files
could be handled this way, but the downside is that the incremental-sync
algorithm would not find matches spanning the chunks.  It would be
interesting to test this and see if the rsync algorithm would be better
served by using a larger number of smaller chunks while segmenting the
file, rather than a smaller number of much larger chunks while
considering the file as a whole.

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