snapshot basics

Can I ask a few basic questions about snapshots;

1) For what purposes do you create a snapshot of a VM?

2) Is there any genuine need to keep them for long periods of time?

3) Are there in risks in keeping them for a long period of time?

4) And I did read that aside from filling up datastores – they can actually impact the performance of the VM for which they are a snapshot of? Is this correct? If so how does the S/S impact the performance of that VM? Or is this a myth?

5) Do you have any sort of policy around snapshots/clearing up of snapshots?
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pma111Asked:
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chris-burnsCommented:
They are a great feature, but should be managed correctly.

1) We use them when testing big changes. If we are applying a large patch then we will shut the server down, snap it then reboot. Should the patch cause issues, a nice reboot works wonders.... For me this is the joy of virtualisation

2) Don't keep them too long, you are better off duplicating them rather than snapping. The reason for this is point 3 / 4

3) the risk is if you snapshot, your running machine then becomes a delta from the snapshot. Which can grow and grow, the longer you keep them the longer it takes to merge the running machine back into the snapshot.

4) there is a performce overhead. as the VMware host must calculate the delta, in the real world any machine snapshot under 2 weeks old has little effect, but we did have one that someone forgot about which was over 2 months old. It made a huge difference when the snapshot was removed.

5) We don't, other than don't leave them too long ... there are tools out there such as vOps which is supposed to help with this, but we have yet to investigate them.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
1. Patching

2. You should not keep them for long periods, not more than 2 days! Performance is affected.

3. Very Big Risks.

4. Filiing up datastores is because of BAD MANAGEMENT

5. Alerts and VMware Administration

please see my EE Article

HOW TO: VMware Snapshots :- Be Patient
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pma111Author Commented:
hancocka please elaborate on 3) very big risks - if you would...

Thanks Chris - no familiar with the term "delta" could you enlighten me in lower tech terms?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
3.

a. Performance is affected.
b. it's not supported or recommended.
c. they get corrupted.
d. if the datastore fills up the VM stops and snapshot gets corrupted.
e. data is lost.

and.....

the VMware Administrator, posts a Question on EE for hancocka to Answer, and fix there Snapshot HELL issues!

Number 1 :- VMware Problem on EE!

Personally I would recommend, DO NOT USE!
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chris-burnsCommented:
The big risks are that the data is in a calculated state. (a delta) some for of data corruption has a bigger risk of wiping out your VM.

Regarding Delta's

So lets say you have a VM that you build from scratch

[Initial VM] - Disk is 1 and 0's

you then snapshot it you then have two "files" or disk images

[Snapshot] - Disk is the 1' and 0's
[VM] - the file contains the changes from now to the snapshot file above.

leave the snapshot and the calculation file gets bigger and bigger... I hope that helps describe a delta. I will see if i can find a better description on the web for you.

Had I designed the system I think I would have made the snapshot the "calculated" file, but I am sure VMware have a valid reason why.
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