During snapshots, where are they saved on the hypervisor, and where do they finally end up?

asked 2015-06-09 08:28:02 -0600

Hey Folks, I'm working with Icehouse backed by NFS for live migrations (so /var/lib/nova/instances/ lives on the nfs mount), and LVM-backed Cinder.

Snapshots never seem to work - they just hang at 0% usually, and I think that it's because they are being staged somewhere while they are being created, and that they are filling up /var temporarily. I've never been able to catch it though to figure out where they are staged.

Could someone please tell me where a snapshot is staged as it's being created, and how large one could expect one to get while it is being created? For example on a 36GB instance with 8GB vRAM and with about 20GB of the instance disk occupied by actual data, would it be 20GB?

When we move to Kilo and use Ceph for cinder and for live migrations, how will that affect the above questions?

Thanks a bunch!

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answered 2015-06-09 09:24:40 -0600

Ranjit gravatar image

updated 2015-06-09 09:26:18 -0600

1. While creating the snapshot , it will access /var/lib/nova/instances/snapshots. It will create temporary file while doing snapshots

2. After that snapshot file will be stored as glance image i.e it will be stored in path used by glance image service. /var/lib/glance/images

As long as these paths are available and accessible, there will be no problem. you can use any backed driver for cinder.

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So the conundrum there is what puts the resultant snapshot (image) into /var/lib/glance/images? They hypervisors have /var/lib/nova/instances mounted, but the hypervisors don't have /var/lib/glance because they aren't glance servers. Does the hypervisor do a glance upload against the controller?

Kodiak Firesmith gravatar imageKodiak Firesmith ( 2015-06-09 09:53:16 -0600 )edit

Yes, you are right . Hypervisor will upload the image to the glance server. Have a look in the nova-compute.log while snapshot process.

Ranjit gravatar imageRanjit ( 2015-06-09 12:02:10 -0600 )edit

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Asked: 2015-06-09 08:28:02 -0600

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Last updated: Jun 09 '15