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Please let me know if it makes any difference in disk space if you take snapshot with or without keeping machine ON?

What is the recommended practice?

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Please provide more context. What kind of snapshots do you want to take (HD images, DB snapshots)? And how will you access the data if the machine is not on? – sleske Feb 22 '10 at 14:56
up vote 3 down vote accepted

It should be faster to do it when it's in a powered off state, and it may use more space depending on which applications you have open. If you do the snapshot while the machine is running, it will have to go through a process called "quiescing".

To quote VMWare:

Quiescing a file system is a process of bringing the on‐disk data of a physical or virtual computer into a state suitable for backups. This process might include such operations as flushing dirty buffers from the operating systems in‐memory cache to disk, or other higher‐level application‐specific tasks.

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