Why does OpenStack distinguish images from snapshots

When a VM has an active snapshot, its virtual disk I/O is not performed on the VM's actual .VMDK files, but instead they are kept unchanged, and whatever changes in the VM is written to different physical files; this allows for the recovery of the previous VM state, but has three important side effects:

• Disk I/O for the VM is much slower.
• Those "delta" files keep growing over time, as more and more disk I/O is performed by the VM.
• When the snapshot is removed, the changes stored in the "delta" files have to be merged back into the main .VMDK files, and this is is very slow and time consuming if the snapshot has been active for a long time.

It is indeed better to not keep active snapshots for a long time. If you need a long-term backup of a VM in a given state, you can just copy the VM somewhere else: this will have no performance impact on the VM, and you'll anyway end up using less disk space than what long-term snapshots would fill up over time.

Also, having a copy of the VM stored in a different place will actually help you if you lose the VM: snapshots are stored together with the VM they belong to, and are only useful if the VM is available; they are totally useless in the case of an actual data loss (like a datastore crash), and thus can't be used as real backups.

Here is some official documentation about snapshots:

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&externalId=1015180

When you create a snapshot, the original disk image is "frozen" in a consistent state, and all write accesses from then on will go to a new differential image. Even worse, as explained here and here, the differential image has the form of a change log, that records every change made to a file since the snapshot was taken. This means, that read accesses would have to read not only one file, but also all difference data (the original data plus every change made to the original data). The number increases even more when you cascade snapshots.