Snapshot confusion

[vmproteau]

For single snapshots, I understand how they work and the less than intuitive information messages when merging or rolling back.

I have a request for what sounds like regularly scheduled snapshots for a client. Keeping multiple snapshots on a VM is something I have avoided but, it appears I won't have a choice.

(1) I'm a little confused about collapsing a big tree of snapshots. Let's say you have 5 (Snap-A, Snap-B, Snap-C, Snap-D, and Snap-E).

(2) If I delete any 1 snapshot, it is merged into it's immediate parent correct? If I delete them all they all are merged back (presumably in order from most recent to oldest).

(3) Reverting seems more confusing. If I revert to Snap-C, my assumption is that Snap-D and Snap-E become irrelevant. What's confusing is they still remain. You have to then delete them seperately. Is there a reason they remain there. I don't see how any downstream snapshots can be used once you've reverted to a parent?

(4) Lastly, besides disk space concerns, are there other issues or guidelines when creating multiple snapshots for a VM like a best practtice upper limit etc.

[AWo]

(1) I'm a little confused about collapsing a big tree of snapshots. Let's say you have 5 (Snap-A, Snap-B, Snap-C, Snap-D, and Snap-E).

(2) If I delete any 1 snapshot, it is merged into it's immediate parent correct? If I delete them all they all are merged back (presumably in order from most recent to oldest).

Deleting a snapshot deletes it and the state it represents, it does not merge it from the logical view. The snashot file is merged as it represents the "here-you-are" state. Because the snapshot file content got merged into the -flat file it is not needed anymore and can be deleted.

AWo

VCP 3 & 4

Author @ vmwire.net

\[:o]===\[o:]

=Would you like to have this posting as a ringtone on your cell phone?=

=Send "Posting" to 911 for only $999999,99!=

[RvdNieuwendijk]

If you are reverting to snapshot Snap-C that means that all the changes since Snap-E are lost and new changes are saved since Snap-C. You can still goto Snap-D or Snap-E but then of course the changes since Snap-C are lost, unless you made a new snapshot before you goto an other snapshot. I don't see a usecase directly, but this might be usefull if you want to test different scenario's.

Beside diskspace, snapshots are also bad for your disk performance. And removing old snapshots make take a lot of time.

LucD made a wonderfull PowerCLI script that gives you a graphical overview of your VM's disks and snapshots. You can find it here.

Robert

[vmproteau]

Verify if I have this straight...

Each snapshot represents a point in time state of a VM. When created, Snap-A refers to this point in time, unchanging file. The actual change file is not really reflected logically in the snapshot manager tree. Creating another snapshot freezes what was the change file which gets reflected as Snap-B and a new change file is created (not reflected in the snapshot manager tree). I continue with Snap-C, Snap-D, and Snap-E.

Reverting to any snapshot at this point deletes the current change file and starts a new one at the reverted level..

[RvdNieuwendijk · Accepted solution]

I think you are right about how snapshots work. But the actual change file is represented by "You are here".

After what I wrote before, I did some testing with snapshots. And it is possible to build a tree of snapshots. Like:

Machine

-Snapshot 1

--Snapshot3

---Snapshot 4

-Snapshot 2

--Snapshot 5

--Snapshot 6

A usecase could be that you want to test with two different versions of the same software, which you cannot install together. Then you can make one snapshot without the software, one snapshot with version 1 and another snapshot with version 2 of the software. With the Snapshot Manager you can easily change between the different versions.

In Scott Lowe's book Mastering VMware vSphere 4 page 348-349 the disadvantages of snapshots are mentioned, which I will summarize here:

1. Snapshots can consume a lot of disk space

2. Snapshots are bad for your disk performance.

3. If the delta disks must grow, the LUNs will be locked, which is bad for the performance of other virtual machines using the same LUN.

4. VMs that have a snapshot cannot use VMware Fault Tolerance.

Robert

Message was edited by: RvdNieuwendijk

[vmproteau]

Thanks Robert. I didn't even think of that possibility yikes. Thanks for the drawback summary. Those are all reason I avoid this type of snapshot use. I will see what they have in mind and decide how to proceed.