Basti's Scratchpad on the Internet
29 Dec 2017

How to set up rsnapshot instead of Time Machine

(This blog post was changed since my initial strategy of disabling the lockfile didn't work. Turns out, the lockfile is required, and backups have to be stacked.)

Yesterday, I wrote about how Time Machine has failed me. Time Machine keeps regular backups, going back as far as your hard drive space permits. In theory. In practice, every year or so it messes up somehow and has to start over, thereby deleting all your older backups. A backup that is not reliable is not a backup.

Luckily, there are alternatives. Probably the easiest is rsync1, a very cool tool that copies files and directories from one place to another. You could simply run this once a day, and have a new backup every day. You can even configure rsync so it doesn't need to copy unchanged files, and instead hard-links them from an older backup. rsnapshot automates this process to keep a number of tiered copies, for example ten hourly backups, seven daily backups, four weekly backups, and a hundred monthly backups. Each backup is then simply a directory that contains your files. No fancy starfield-GUI, but utterly reliable and trivial to understand 2.

Setting up rsnapshot on macOS is not quite as straight-forward as I'd like, and I couldn't find a great guide online. So, without further ado, here's how to configure rsnapshot on macOS:

Caveats

The configuration file of rsnapshot says that you might experience data corruption if you run several copies of rsnapshot at the same time (and you can use the lockfile to prevent this). This is a problem if your computer is asleep while rsnapshot is scheduled to run, since launchd will then re-schedule all missed tasks at once when the computer wakes up. If you enable the lockfile, only one of them will run.

On the other hand, only the hourly task will actually create a new backup. All higher-level backup tiers merely copy existing backups around, so in theory, they shouldn't step on each other's toes when run concurrently. I have opened an issue asking about this.

There are other possible solutions: ① You could modify the launchd entry such that backups only trigger after a few minutes or, better yet, only once all other instances of rsnapshot have finished. I am not sure if launchd supports this, though. ② You could schedule the hourly task using cron instead of launchd, since cron will not reschedule missed tasks. This would only work for two tiers of backups, though. ③ You could just ignore the issue and hope for the best. After all, if a daily or hourly backup gets corrupted every now and then, you still have enough working backups…

Footnotes:

1

rsync is one of those reliable tools I talked about. It is rock solid, incredibly versatile, and unapologetically single-minded. A true gem!

2

This works great for local backups. If you need encrypted backups or compressed backups (maybe on an untrusted remote machine), this post recommends Borg instead of rsnapshot, but you will lose the simplicity of simple directories.

3

I use launchd instead of cron since launchd will re-schedule missed backups if the computer was asleep.

4

This will fail if the hourly backup takes longer than 15 minutes. This is rather unlikely, though, or at least should not happen often enough to be of concern.

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