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VMware Snapshots and Storage Use

Points of My Scenario:
1. I am vSphere and Windows update admin for 100+ Windows virtual machines.
2. I need VMware snapshots as a roll-back plan for VMs that fail from software updates.
3. I must ensure adequate free space on data stores to handle snapshots.
4. Snapshots will include virtual memory.
QUESTION:
Is the storage used equal to VM size at point of snapshot PLUS changes due to software updates?
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waltforbes
Asked:
waltforbes
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1 Solution
 
Casey WeaverNetwork EngineerCommented:
Snapshot size is equal to 0+RAM. The snapshot typically starts at 100MB and continues to rapidly grow with each disk block change. So if you have each VM at 4GB of RAM, you could round and say that after updates (depending on how much you're updating) you could be looking at 500-600GB of storage usage for 100 VM's to be updated via snapshots. Obviously, RAM allocation will make this figure grow much larger much faster!
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waltforbesAuthor Commented:
To Heymrdj:
You state that "snapshot size equal to zero+RAM" - does this mean that initially, only the contents of RAM is copied to disk when snapshot is taken?
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Casey WeaverNetwork EngineerCommented:
Correct. There is about 100MB or so given to the log start, but other than that, at initialization (and quiescence of the OS for the snapshot) the log is "zero" plus the active RAM state. If you snapshot when the OS is off your snapshot will be minimal. But every block change afterwards is added to that log, hence how quickly it grows.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1025279
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waltforbesAuthor Commented:
To Heymrdj:
Did you imply the following: if I snapshot a VM that is off, the snapshot will be minimal (for log start)...BUT... as soon as I power up the OS, the snapshot grows to the size of the RAM. Correct?
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Casey WeaverNetwork EngineerCommented:
Correct. That is if you set your snapshots to do RAM as well. For the most part it targets the filesystem, which obviously RAM is filled via the file system, but yes it gets RAM as well if it is configured as so (in my experience that is by default).
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waltforbesAuthor Commented:
To Heymrdj:
Final question: what is/are the caveat(s) to snapshot a VM without the RAM?
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Casey WeaverNetwork EngineerCommented:
Your rollback would only be to shutdown states, which whatever software you're running might not like. I *always* recommend a fully quiesced RAM included snapshot if you're going to snapshot a windows machine, or especially a windows server. There's so many small databases and processes running on a fully roled server it can be impossible to straighten them out after a failed snapshot reversion.
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waltforbesAuthor Commented:
HeyMrDJ: you have tremendously enhanced my understanding of snapshots! My decision-making is much clearer and confident. I am so appreciative. Thank you ever so much!
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