January 25th, 2011
rdiff-backup vs. ZFS
As I’ve been writing about backups, I’ve gone ahead and run some tests with rdiff-backup. I have been using rdiff-backup personally for many years now — probably since 2002, when I packaged it up for Debian. It’s a nice, stable system, but I always like to look at other options for things every so often.
rdiff-backup stores an uncompressed current mirror of the filesystem, similar to rsync. History is achieved by the use of compressed backwards binary deltas generated by rdiff (using the rsync algorithm). So, you can restore the current copy very easily — a simple cp will do if you don’t need to preserve permissions. rdiff-backup restores previous copies by applying all necessary binary deltas to generate the previous version.
Things I like about rdiff-backup:
- Reasonably space-efficient, especially where history is concerned
- Easily scriptable and nice CLI
- Unlike tools such as duplicity, there is no need to periodically run full backups — old backups can be deleted without impacting the ability to restore more current backups
Things I don’t like about it:
- Speed. It can be really slow. Deleting 3 months’ worth of old history takes hours. It has to unlink vast numbers of files — and that’s pretty much it, but it does it really slowly. Restores, backups, etc. are all slow as well. Even just getting a list of your increment sizes so you’d know how much space would be saved can take a very long time.
- The current backup copy is stored without any kind of compression, which is not at all space-efficient
- It creates vast numbers of little files that take forever to delete or summarize
So I thought I would examine how efficient ZFS would be. I wrote a script that would replay the rdiff-backup history — first it would rsync the current copy onto the ZFS filesystem and make a ZFS snapshot. Then each previous version was processed by my script (rdiff-backup’s files are sufficiently standard that a shell script can process them), and a ZFS snapshot created after each. This lets me directly compare the space used by rdiff-backup to that used by ZFS using actual history.
I enabled gzip-3 compression and block dedup in ZFS.
My backups were nearly 1TB in size and the amount of space I had available for ZFS was roughly 600GB, so I couldn’t test all of them. As it happened, I tested the ones that were the worst-case scenario for ZFS: my photos, music collection, etc. These files had very little duplication and very little compressibility. Plus a backup of my regular server that was reasonably compressible.
The total size of the data backed up with rdiff-backup was 583 GB. With ZFS, this came to 498GB. My dedup ratio on this was only 1.05 (meaning 5% or 25GB saved). The compression ratio was 1.12 (60GB saved). The combined ratio was 1.17 (85GB saved). Interestingly 498 + 85 = 583.
Remember that the data under test here was mostly a worst-case scenario for ZFS. It would probably have done better had I had the time to throw the rest of my dataset at it (such as the 60GB backup of my iPod, which would have mostly deduplicated with the backup of my music server).
One problem with ZFS is that dedup is very memory-hungry. This is common knowledge and it is advertised that you need to use roughly 2GB of RAM per TB of disk when using dedup. I don’t have quite that much to dedicate to it, so ZFS got VERY slow and thrashed the disk a lot after the ARC grew to about 300MB. I found some tweakables in zfsrc and the zfs command that let me tweak the ARC cache to grow bigger. But the machine in question only has 2GB RAM, and is doing lots of other things as well, so this barely improved anything. Note that this dedup RAM requirement is not out of line with what is expected from these sorts of solutions.
Even if I got absolutely stellar dedup ratio of 2:1, that would get me at most 1TB. The cost of buying a 1TB disk is less than the cost of upgrading my system to 4GB RAM, so dedup isn’t worth it here.
I think the lesson is: think carefully about where dedup makes sense. If you’re storing a bunch of nearly-identical virtual machine images — the sort of canonical use case for this — go for it. A general fileserver — well, maybe you should just add more disk instead of more RAM.
Then that raises the question: if I don’t need dedup from ZFS, do I bother with it at all, or just use ext4 and LVM snapshots? I think ZFS still makes sense, given its built-in support for compression and very fast snapshots — LVM snapshots are known to cause serious degradation to write performance once enabled, which ZFS doesn’t.
So I plan to switch my backups to use ZFS. A few observations on this:
- Some testing suggests that the time to delete a few months of old snapshots will be a minute or two with ZFS compared to hours with rdiff-backup.
- ZFS has shown itself to be more space-efficient than rdiff-backup, even without dedup enabled.
- There are clear performance and convenience wins with ZFS.
- Provide scheduling, making sure that we don’t hammer the server with 30 clients all at once
- Provide for “run before” jobs to do things like snapshot databases
- Be silent on success and scream loudly via emails to administrators on any kind of error… and keep backing up other systems when there is an error
- Create snapshots and provide an automated way to remove old snapshots (or mount them for reading, as ZFS-fuse doesn’t support the .zfs snapshot directory yet)
So now comes the question of backup scripts. rsync is obviously a pretty nice choice here — and if used with –inplace perhaps even will play friendly with ZFS snapshots even if dedup is off. But let’s say I’m backing up a few machines at home, or perhaps dozens at work. There is a need to automate all of this. Specifically, there’s a need to:
To date I haven’t found anything that looks suitable. I found a shell script system called rsbackup that does a large part of this, but something about using a script whose homepage is a forum makes me less than 100% confident.
On the securing the backups front, rsync comes with a good-looking rrsync script (inexplicably installed under /usr/share/doc/rsync/scripts instead of /usr/bin on Debian) that can help secure the SSH authorization. GNU rush also looks like a useful restricted shell.