Re: Explaining snapshots (for backup)
On Tue, 15 Nov 2022 09:40:11 -0500
Dan Ritter <dsr@randomstring.org> wrote:
> rhkramer@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
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