[Borgbackup] status of cache resync improvements

[Dan Christensen]

I read various issues and pull requests about changes to the cache
resync process in borg, and was wondering what the current status is.
I have multiple machines backing up to one repo, and the resyncs 
with attic are getting very slow.  I also prune fairly regularly.

As an example with attic, one repository I have has

                       Original size      Compressed size    Deduplicated size
All archives:                1.61 TB            976.84 GB             30.92 GB

This repo contains 188 archives.

Rebuilding the cache, over gigabit ethernet, takes 40 minutes, and is
CPU bound on the local machine.  The local and remote machines both have
plenty of ram and the local machine has a fast cpu.  (For some reason, a
similar but slightly smaller repo I have needs 2 hours for a cache
rebuild, on the same machines.)

Attic's cache directory is 98MB, and it takes 2.3 *seconds* to copy it
from the remote machine to the local machine using scp.

Because of this, it seems to me to make sense to keep a copy of the
cache in the remote repo, and then copy to the local machine when borg
notices that the cache is out of sync.  The cache would add 0.3% to the
repo size, in this case.

But maybe the improvements that borg has made make things fast enough
that this isn't needed?

Thanks,

Dan