On 2022-11-10 at 09:06, Dan Ritter wrote: > Now, RAID is not a backup because it is a single store of data: if > you delete something from it, it is deleted. If you suffer a > lightning strike to the server, there's no recovery from molten > metal. Here's where I find disagreement. Say you didn't use RAID, and you had two disks in the same machine. In order to avoid data loss in the event that one of the disks failed, you engaged in a practice of copying all files from one disk onto the other. That process could, and would, easily be referred to as backing up the files. It's not a very distant backup, and it wouldn't protect against that lightning strike, but it's still a separate backed-up copy. But copying those files manually is a pain, so you might well set up a process to automate it. That then becomes a scheduled backup, from one disk onto another. That scheduled process means that you have periods where the most recently updated copy of the live data hasn't made it into the backup, so there's still a time window where you're at risk of data loss if the first disk fails. So you might set things up for the automated process to in effect run continuously, writing the data to both disks in parallel as it comes in. And at that point you've basically reinvented mirroring RAID. You've also lost the protection against "if you delete something from it"; unlike deeper, more robust forms of backup, RAID does not protect against accidental deletion. But you still have the protection against "if one disk fails" - and that one single layer of protection against one single cause of data loss is, I contend, still valid to refer to as a "backup" just as much as the original manually-made copies were. > Some filesystems have snapshotting. Snapshotting can protect you > from the accidental deletion scenario, by allowing you to recover > quickly, but does not protect you from lightning. > > The lightning scenario requires a copy of the data in some other > location. That's a backup. There are many possible causes of data loss. My contention is that anything that protects against *any* of them qualifies as some level of backup, and that there are consequently multiple levels / tiers / degrees / etc. of backup. RAID is not an advanced form of protection against data loss; it only protects against one type of cause. But it still does protect against that one type, and thus it is not valid to kick it out of that circle entirely. -- The Wanderer The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man. -- George Bernard Shaw
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