[Users] Basic questions about ploop snapshotting

[Devon B.]

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:sgbarrett at 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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