Famously, one difference between HDDs and SSDs is that SSDs have limits on how much data you can write to them and HDDs mostly don't (which means that SSDs have a definite lifetime). These limits are a matter both of actual failure and of warranty coverage, with the warranty coverage limit generally being lower. We don't normally think about about this, though, because we're not a write-intensive place. Sometimes there are surprises, such as high write volume on our MTAs or more write volume than I expected on my desktops, but even then the absolute numbers tend to be low and not anywhere near the write endurance ratings of our SSDs.

Recently, though, we realized that we have one place with high write volume, more than high enough to cause problems with ordinary SSDs, and that's on our Amanda backup servers. When an Amanda server takes in backups and puts them on 'tapes', it first writes each backup to a staging disk and then later copies from the staging disk to 'tape' (in our Amanda environment, these are HDDs). If you have a 10G network and fileservers with SATA SSDs, as we do, how fast an ordinary HDD can write data generally becomes your bottleneck. If your fileservers can provide data at several hundred MBytes/sec and Amanda can deliver that over the network, a single HDD staging disk or even a stripe of two of them isn't enough to keep up.

However, the nature of the work that a staging disk does means that it sees high write volume. Every day, all of your backups sluice through the staging disk (or disks) on their way to 'tapes'. If you back up 3 TB to 4 TB a day per backup server, that's 3 TB to 4 TB of writes to the staging disk. It would be nice to use SSDs here to speed up backups, but no ordinary SSD has that sort of write endurance. Much as you'd have to aggregate a bunch of HDDs to get the write speed you'd need, you'd have to aggregate a bunch of ordinary SSDs to get any individual one down to the write endurance level they can survive.

(In a way the initial backup to the staging disks is often the most important part of how fast your backups are, because that's when your other machines may be bogged down with making backups or otherwise affected by the process.)

There are special enterprise SSDs with much higher write endurance, but they also come with much higher price tags. For once, this extra cost is not just because the e word has been attached to something. The normal write endurance limits are intrinsic to how current solid state storage cells work; to increase them, either the SSD must be over-provisioned or it needs to use more expensive but more robust cell technology, or both. Neither of these is free.