I believe a single file does exist for the snapshot but it isn't what
you think. I believe, if ploop snapshotting is what I believe it to be,
that file is only used to log the changes to the filesystem while
preserving the root.hdd file (your original filesystem). Once a
snapshot is created, the original root.hdd goes untouched, any changes
to the filesystem go into the file designated for the snapshots
(root.hdd.uuid). Then when a new snapshot file is created, the
previous snapshot file is preserved and the new snapshot file gets all
the changes. The top delta file keeps track of the snapshots.
Simon Barrett <mailto:sgbarr...@gmail.com>
Tuesday, November 18, 2014 6:02 AM
> I'm not quite following all of your statements. Specifically, I don't
> see why you think you need a base AND a first snapshot. One snapshot is
> sufficient for later restoring. Also, I don't think you need to
> understand the underlying architecture in order to successfully use
> snapshots (it might be helpful or interesting, but not really
> necessary). The command 'snapshot-switch' reliably restores the state of
> the CT it had at the time when you created the snapshot. It's as simple
> as that. Also, when deleting snapshots, vzctl auto-magically does the
> right thing in that it merges or deletes when it is appropriate without
> affecting the other snapshots of the given container.
>
Sorry about kicking this off again, but this hit to me over the
weekend and it cleared things up for me. Please, someone, correct me
if I'm wrong.
As I understand it now, no single file corresponds to a snapshot and
to think of it that way will lead you to do something silly with your
data. A snapshot is an event. If you want to think of it in terms of
files, it's the gap between the root.hdd and the delta. It's a like a
HUP in I/O gives you a spot to go back to. When you delete a
snapshot, you're deleting the event and this means the files that are
either side of that discontinuity will be merged/healed.
When you mount a snapshot, for example when doing file based backup
(https://openvz.org/Ploop/Backup) you're not actually mounting a
snapshot because there is no snapshot file to mount; you're mounting
the pre-snapshot data.
Simon
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