Is Amazon AWS/EBS snapshotting just LVM, or what?

[Tom Buskey]

On Thu, Sep 28, 2017 at 2:00 PM, Joshua Judson Rosen <rozzin at hackerposse.com
> wrote:

> On 09/28/2017 01:46 PM, Tom Buskey wrote:
> > I work with OpenStack.  It manages images in Glance which sit above its
> object storage, Swift.
> >
> > On the POC clouds, you can use LVM as a backend for Glance.
> Snapshotting is *very* slow.  30 minutes for a snap of a
> > 80GB VM that's shutdown.
>
> OK..., that surprises me. A lot.
>


>
> For comparison, I just made an LVM snapshot of a volume 50% larger than
> that, that's *in use*
> (and mostly not in cache, if that even makes a difference, since my
> buffer+cache shows as only 17GB *total*),
> and the whole operation took only a fraction of a second:
>
>         rozzin at zuul:~ $ time sudo lvcreate --name home_snap --size 128G
> --snapshot zuul-vg/home
>           Using default stripesize 64.00 KiB.
>           Logical volume "home_snap" created.
>
>         real    0m0.349s
>         user    0m0.028s
>         sys     0m0.060s
>
>
> How in the world does that translate to 30-minutes (*5 thousand* x time)
> for a volume only 0.63x as big?
>
> When you say "snapshotting on top of LVM", does that entail actually
> making a full copy
> after the LVM snapshot is made--or something like that?
>
>
I'm not exactly sure what OpenStack VM snapshots are doing under the
covers.  I'm sure it's not an LVM snapshot.  Openstack VMs use .qcow2
files, which are COW.  There is a copy/convert process in the Openstack
process so the image the VM uses can be resumed and you can redeploy the
snapshot as a new VM or even copy/launch it on another system.

I misspoke about LVM for Glance/Swift.  The backend for the images are on
top of a filesystem in the POC clouds.  LVM is used for Cinder, the block
image store.  Ceph is often used to drop in replace LVM for Cinder and
files for Swift objects.
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