Tag Archives: unison

High-availability /home revisited

About a month ago, I wrote about my experiments in ways to keep my home directory consistently available. I ended up concluding that DRBD is a neat solution for true high-availability systems, but it’s not really worth the trouble for what I want to do, which is keeping my home directory available and in-sync across several systems.

Considering the problem more, I determined that I really value a simple setup. Specifically, I want something that uses very common software, and is resistant to network failures. My local network going down is an extremely rare occurence, but it’s possible that my primary workstation will become a portable machine at some point in the future- if that happens, anything that depends on a constant network connection becomes hard to work with.

If an always-online option is out of the question, I can also consider solutions which can handle concurrent modification (which DRBD can do, but requires using OCFS, making that solution a no-go).

Rsync

rsync is many users’ first choice for moving files between computers, and for good reason: it’s efficient and easy to use.  The downside in this case is that rsync tends to be destructive, because the source of a copy operation is taken to be the canonical version, any modifications made in the destination will be wiped out.  I already have regular cron jobs running incremental backups of my entire /home so the risk of rsync permanently destroying valuable data is low.  However, being forced to recover from backup in case of accidental deletions is a hassle, and increases the danger of actual data loss.

In that light, a dumb rsync from the NAS at boot-time and back to it at shutdown could make sense, but carries undesirable risk.  It would be possible to instruct rsync to never delete files, but the convenience factor is reduced, since any file deletions would have to be done manually after boot-up.  What else is there?

Unison

I eventually decided to just use Unison, another well-known file synchronization utility.  Unison is able to handle non-conflicting changes between destinations as well as intelligently detect which end of a transfer has been modified.  Put simply, it solves the problems of rsync, although there are still situations where it requires manual intervention.  Those are handled with reasonable grace, however, with prompting for which copy to take, or the ability to preserve both and manually resolve the conflict.

Knowing Unison can do what I want and with acceptable amounts of automation (mostly only requiring intervention on conflicting changes), it became a simple matter of configuration.  Observing that all the important files in my home directory which are not already covered by some other synchronization scheme (such as configuration files managed with Mercurial) are only in a few subdirectories, I quickly arrived at the following profile:

root = /home/tari
root = /media/Caring/sync/tari

path = incoming
path = pictures
path = projects
path = wallpapers

Fairly obvious function here, the two sync roots are /home/tari (my home directory) and /media/Caring/sync/tari (the NAS is mounted via NFS at /media/Caring), and only the four listed directories will be syncronized. An easy and robust solution.

I have yet to configure the system for automatic syncronization, but I’ll probably end up simply installing a few scripts to run unison at boot and when shutting down, observing that other copies of the data are unlikely to change while my workstation is active.  Some additional hooks may be desired, but I don’t expect configuration to be difficult.  If it ends up being more complex, I’ll just have to post another update on how I did it.

Update Jan. 30: I ended up adding a line to my rc.local and rc.shutdown scripts that invokes unison:

su tari -c "unison -auto home"

Note that the Unison profile above is stored as ~/.unison/home.prf, so this handles syncing everything I listed above.