pa...@quillandmouse.com wrote: > On Tue, 15 Nov 2022 09:40:11 -0500 > Dan Ritter <d...@randomstring.org> wrote: > > > rhkra...@gmail.com wrote: > > > I'm not really clear on the concept of a snapshot (for backup) -- > > > I've done a little googling but haven't found an explanation that > > > "satisfies" me. > > > > > > Starting from a beginning, I suppose I could copy the entire > > > contents of whatever I wanted to make a snapshot of (by any of a > > > variety of tools -- dd, cp, ...) and call that a snapshot, although > > > the more common name for it would be a "full backup". > > > > Let's look at the larger circumstances. > > > > In ordinary usage, there are tens to thousands of processes > > runnning on your system. Some of them are emitting logs or > > writing files. > > > > Taking a backup takes some time. During that time, some files > > get written, some get opened, and some are related to each other > > (by the processes) in ways which are inconsistent until all of > > them are written. > > > > A snapshot differs from a backup in two important regards: > > > > - first, it requires the filesystem to bring writes to a halt. > > There is now a consistent view. > > > > - second, it doesn't actually copy things. It just records their > > state and, when done, allows future writes to continue -- writes > > which are not part of this snapshot. > > > > As a result, you can take a snapshot and then: > > > > - discard it (trivial) > > > > - look through it and copy off any file or group of files, thus > > getting what they contained at the time of the snapshot, not the > > what they contain now (excellent for recovering from an > > accidental delete) > > > > - copy all of it off elsewhere, producing a consistent full > > backup. > > > > Assuming a snapshot is taken so that you can recover a filesystem to a > previous state (or the current state). Is that correct?
Yes. > I don't understand "recording the state" of files. To me, this means > the ownership, size, etc., not the contents. That doesn't seem valuable > for recovering the state of a system. That's the metadata. A snapshot fixes the data as well as the metadata. State is everything written to disk. > Let's assume, as the OP says, you do an original full backup. A > snapshot ought to record either the contents of all the files which > have changed, or record the delta of each file which has changed. > Thus, you'd be able to recover a filesystem to either some prior state > or its current state, using the snapshot. > > Am I missing something? Well, you don't need to recover it to the current state, because you already have that. But, yes, a snapshot from yesterday allows you to copy from that snapshot any or all changed files to the current filesystem. -dsr-
