Wandering Thoughts archives

What affects how fast you can restore backups

I was asked today if I thought that a disk-based backup system could do restores faster than a tape-based system. My best answer was a 'maybe', because it really depends on what the limiting factor is in your restores. Let's look at all of the things that have to happen in a restore:

1. you find and load the tape into the tape drive or tape library, if it's not already there.
2. if you have a tape library, it loads the tape into the tape drive.
3. position the tape to the right dump image.
4. read through the dump image until you get to the data that you want to restore.
5. transport the data over the network to the target system that you're restoring on.
6. write the data out on the target system.

Of these activities, a disk-based backup system makes the second and third basically instantaneous and may speed up reading the dump image from the media. It can't do anything about the speed of the network or how fast the target system can write things to disk, or how long you take to find and retrieve the right media.

It's also worth noting that your backup system can make a difference in this, depending on what the limiting factors are. For example, Amanda runs the restore command on the target system, which means that step four requires transporting the entire dump image over the network to the target system.

And speaking of media read speeds, one advantage disks have is that it is less tricky to get good read speeds. Because of mechanical issues, tape drives often have a minimum read speed that's necessary to get good performance; if you drop below that speed, the read performance goes significantly down because the tape drive has to stop and restart all the time (known as shoe-shining).

Another advantage of disk-based backup systems

One of the slightly subtle advantages of disk-based backup systems over tape-based backup systems is that capacity expansion is much easier; all you have to do is start using bigger disks. Since SATA is SATA, you don't need to replace your enclosure, and it is relatively easy to have multiple generations of disks with different capacities cycling through your system.

Contrast this with tape backups. To upgrade to higher capacity tapes, you have to not just buy the tapes, you generally have to buy an entire new tape drive that can write the new high-capacity format, for much more money and often a much more complex environment (at least in the old days, new tape drives were often not truly happy writing to the old, lower-capacity tapes you already had, and sometimes couldn't do it at all).

Fundamentally, what's going on here is that tape has a made a tradeoff; it has put the work and the smarts in the tape drive instead of the media, so that the media is cheap and simple while the tape drive is expensive and complex. Modern hard drives have gone the other way; the hard drive is ferociously complex (and is only cheap because they are made in such bulk), while the interface is relatively simple and general.

(There are subtle advantages to the tape tradeoff; for example, the simplicity means that there is less to get broken in tape media. And the tradeoff is a great deal if you have a lot of tapes compared to how many tape drives you have.)