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OpenStack® Glance: The Treachery of Images (Apologies to Rene Magriette)


Guest Post: Rick Evans, Sr Evangelism Technologist, HP Helion


What is this “image” of which you speak? “Snapshots are not images.”


In my experience, that distinction is not well understood. This official terminology around Glance seems to a be more complicated than it needs to be. As is typical with users of jargon, I find speakers and writers who have been working with the OpenStack® for a long time use this terminology as if their audience already knows their special language. The official terminology is technically correct, but I think it is unnecessarily confusing to people trying to learn OpenStack technologies. Hopefully the following explanation will help.


snapshot.jpgMost people who are familiar with virtualization, know that you can take “a snapshot” of a Virtual Machine. You can use that snapshot to instantiate a new virtual machine in the exact state it was in when the snapshot was taken. Technically, apparently, you do not instantiate virtual machine from a “snapshot.” You instantiate a virtual machine from an “image” … which, typically, is based on a snapshot. Get it? “Snapshots are not images.”


Technically, a snapshot is only a copy of the disk contents at the time of the snapshot. If there is data in buffers that has not yet been written to disk, you cannot restore a true copy of that virtual machine at the time the snapshot was taken.


From the OpenStack glossary:



A collection of files for a specific operating system (OS) that you use to create or rebuild a server. OpenStack provides pre-built images. You can also create custom images, or snapshots, from servers that you have launched. Custom images can be used for data backups or as "gold" images for additional servers.




A point-in-time copy of an OpenStack storage volume or image. Use storage volume snapshots to back up volumes. Use image snapshots to back up data, or as "gold" images for additional servers.


Further reference:

 Image:  From

Senior Manager, Cloud Online Marketing
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About the Author


I manage the HPE Helion social media and website teams promoting the enterprise cloud solutions at HPE for hybrid, public, and private clouds. I was previously at Dell promoting their Cloud solutions and was the open source community manager for OpenStack and at Rackspace and Citrix Systems. While at Citrix Systems, I founded the Citrix Developer Network, developed global alliance and licensing programs, and even once added audio to the DOS ICA client with assembler. Follow me at @SpectorID

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