multiple sessions to same destination
bet at rahul.net
Thu Oct 10 20:54:00 EST 2002
2002-10-10-16:09:16 Derek Simkowiak:
> I'd like to know what you mean by "rsync handles it fairly well".
> Any information you have would be greatly appreciated.
I think I can help a little here.
Each rsync at the destination has its own separate remote rsync
process whacking on the file.
Rsync doesn't whack directly in the target file; rather, the remote
rsync process uses some combination of wads of data read from the
target file plus wads of data passed from the sending rsync, to
assemble a new copy; it's assembled in a tmp file and moved into
The rsync opens the target file to read; if some other rsync moves a
new file into place before that, there's no concurrency, this is
pure sequential rsyncs; if it moves the target file into place after
it's been opened, the older copy of the target will still be used by
the process we're looking at, through the open file handle it holds;
the intruding copy won't have any effect.
The tmp files used to build the new copies have some magic juju dust
sprinkled on the filename; while I've not analyzed them or looked at
the code to figure out where the magic comes from, certainly
collisions here are improbable if not formally impossible.
All in all, it's pretty safe to have multiple rsyncs aimed at the
same destination file; you'll get one of 'em, intact. And I think
it's even possible that if every invoker uses --backup with a
different --backup-dir, it's even possible that rsync might
guarantee the safe archival of _every_ copy that's sent. I wouldn't
bet against it, rsync seems to be some pretty serious engineering.
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