Re: Unison hangs on copy

[Randy Kramer <rhkramer@gmail.com>]

On Saturday 23 July 2011 08:25:10 am Roger Leigh wrote:
> I use unison several times a day to sync over 100GiB of data.
> I see it stall quite frequently.  I think it's a bug in unison,
> possibly timing related.  I see it only when syncing over the
> internet on a DSL connection, not over a LAN.

Don't know if this helps, but if unison uses rsync (which, iirc, someone 
mentioned it does), you should be aware that rsync has quite a bit 
of "overhead" (in some sense) to determine exactly which pieces of a 
file are different and then transmits only those.  

So, again, iirc, it does calculations both at the rsync server and the 
rsync client to get "checksums" (not quite the word I'm looking for, 
but close) of pieces of the file on both ends, then exchanges those 
checksums to see where differences exist, then exchanges the necessary 
pieces of the file, and finally reassembles the file.

With very large files, it is not surprising to see the link quiet for 
periods of time while calculations are being done at each end.

(I once read the academic thesis that the author of rsync wrote about 
the rsync algorithm--I don't accurately remember the details, but what 
I wrote above gives the general idea.)

Randy Kramer