Clarification for Snapshots

[paulzikopoulos]

Could I get some help here. OK so I believe a snapshot is a Point In Time (PIT) backup. OK. I get that.

If I have 3 snapshots are they all PIT full restorable backups or does Snapshot N need Snapshot N-1 + Snapshot N-2?

So here is what I really want to get after. I have a certain speech I do and it takes about 15 command line windows on Ubuntu all opened and configured and then I go through and run them.

Is there a way I can snapshot this when I want to run it, but not go back permanently in time like that. So Snapshot N-2 is that one. I want to do more snapshots N+1, N+2 but when I have that one demo, go back to N-2 and discard it, so I don't restore permanently to the point in time.

Is this possible or should I just copy a paused VMWare state with the windows open, or with the Snapshotted and restore ion another session?

[wila]

Hi,

I'm not quite sure what you are trying to say, but let's start at the beginning.

OK so I believe a snapshot is a Point In Time (PIT) backup. OK. I get that.

Nope. It's not a backup. When you create a snapshot, you can return to that point in time because:

- your virtual disk is basically stunned and marked read only from there on.

- your RAM contents is written to disk

- your CPU/GPU and other hardware state is written out to disk

- a new Copy on Write disk is created that will store all differences stored to the virtual disk that is now readonly. This is now your new virtual disk.

Every time you create a snapshot, the above number of events happen, so you'll end up with a chain of virtual disks.

If you miss a virtual disk in the chain.. your VM is dead and has some or even complete data loss, depending on what part of the chain is gone.

You will need advice of a data recovery specialist though.

Anyways..

You can create a tree of your snapshot chain. IOW something like this:

--> Snap1 ----> Snap2 ---- Snap3

         \---------> Snap4

Both Snap2 as well as Snap4 continue from the state of the virtual machine at Snap1

Say no longer need the state of Snap2 then you can delete/commit that snapshot and the snapshot tree becomes like this:

--> Snap1 ----> Snap3

         \---------> Snap4

The way it works is as if Snap2 never was taken.

For demo and testing purposes it is fine to have a complex snapshot tree.

If OTOH you have production data in your VM then it is recommended to delete your snapshots as fast as possible as the more snapshots you have, the more vulnerable your VM is to data loss.

note: delete=commit. They mean the same thing.

Hope this helps a bit.

--

Wil

[paulzikopoulos]

Thanks for the reply.

I should have clarified, I meant act like a delta backup.

My Snap5 did you mean Snap4?

I get the more snapshots more vulnerable now for sure - thanks for that.

I can't seem top find how I create a split snapshot (Snap 4)

So I think you are saying if I follow below I could do this

SESSION --- DEMO LIVE (Snapshot A)

          | -------------------||--------------------- Snapshot B ---------------- Snapshot C -------------- CURRENT STATE

The I could also flash back to Snapshot A to do the demo and then when I want to get back I what, just boot it up again OR I make CURRENT STATE a snapshot. Then I prune B and C as I  add more and everytime I want to run my demo I just pretty much snapshot the current state and then go back to Snapshot A which is forkd, then recover to Current State snapshot I take before the demo

Make sense?

Or perhaps I should just write a script to open all the windows ... but it's good for me to learn this

Do I have this right? Thanks for your help, now just to figure out how to make that fork, I don't see it on right-click options

[wila]

Hi,

Yes I meant Snap4 when I said Snap5, I have corrected my post above.

You make a fork by going back to a snapshot in time and continue working there.

As soon as you create a new snapshot from that point on you have created a fork.

--

Wil