[Openstack] Understanding "flavors" of VM

Lei Zhang zhang.lei.fly at gmail.com
Thu Dec 6 05:28:27 UTC 2012

Hi Michael,

Could you send us the doc link? Thanks a lot.

On Thu, Dec 6, 2012 at 6:47 AM, Michael Still
<michael.still at canonical.com>wrote:

> On 12/05/2012 06:59 PM, Marco CONSONNI wrote:
> > To be honest it seems like I missed something because, from your
> > investigation, the storage is kept under _base. Strange. I didn't know
> that.
> Hi! The following description is libvirt specific. Bearing in mind that
> this code is a moving target and has been re-written at least three
> times in the last year [1], it works a bit like this...
> - you request an instance using a given image
> - that image is fetched to _base from glance (if its not already there)
> - that image is format converted if required, and then resized to the
> requested size
> - the instance disk image is a copy-on-write layer on top of that
> resized image in _base and is stored in the instance's directory
> - the instance is booted using the COW layer
> If another instance with the same image / disk size starts, it can short
> circuit the process and use the already existing image, which is cool.
> When an instance is terminated, the instance directory is removed, but
> files in _base remain.
> If you have image cache management turned on, then the files in _base
> are periodically cleaned up. The files in _base are also checksummed to
> try and detect file corruption, although that hasn't been the most loved
> feature ever implemented.
> Hope this helps,
> Michael
> 1: Several times I have gone to write a blog post about how this works,
> and then realized the code has changed again.
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Lei Zhang

Blog: http://jeffrey4l.github.com
twitter/weibo: @jeffrey4l
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