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? 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. 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? Paul -- Paul M. Foster Personal Blog: http://noferblatz.com Company Site: http://quillandmouse.com Software Projects: https://gitlab.com/paulmfoster
