[Dirvish] btrfs support: v3 / "cow" option v1

[Adrian von Bidder]

Heyho!

I had a few moments to spare recently...

Summary: support using btrfs snapshots instead of hardlinked rsync trees. 
Also uses the --inplace option to rsync, so for changed files only the 
modified parts will be duplicated. (Hmm. rsync doesn't use fallocate(2) to 
punch holes in files yet, I assume, so there's room for improvement here for 
cases where a piece of a file has been cut out, as in a video stream.)

Also: a (first?) batch of patches that support data deduplication has been 
posted to the btrfs mailing list in January, so btrfs will be even more 
interesting in the medium future for those who use dirvish to backup a 
number of similar machines to a central backup machine. Not sure when this 
will be really available though...

Change vs. the first versions: as was proposed in the original discussion of 
this, the "btrfs" option has now been renamed into "cow" (copy on write); 
supported values are 0 (default) and btrfs. Supporting other cow filesystems 
is left as an exercise to the reader.

Status: while this is running here, I've only just written this code a few 
days ago, so some testing / feedback is welcome.

Since I expect there are not that many users of this yet, no migration path 
for old vaults is provided, I didn't want to bloat the code with support 
code for stuff that wasn't even released. Just insert "cow: btrfs" into all 
summary files (and default.conf, of course) and you're fine.

cheers
-- vbi

-- 
Could this mail be a fake? (Answer: No! - http://fortytwo.ch/gpg/intro)
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