[openstack-dev] [nova] Question about boot-from-volume instance and flavor

Rui Chen chenrui.momo at gmail.com
Tue Mar 3 09:10:24 UTC 2015

Hi all,

When we boot instance from volume, we find some ambiguous description about
flavor root_gb in operations guide,

*Virtual root disk size in gigabytes. This is an ephemeral disk the base
image is copied into. You don't use it when you boot from a persistent
volume. *
*The "0" size is a special case that uses the native base image size as the
size of the ephemeral root volume.*

'You don't use it(root_gb) when you boot from a persistent volume.'
It means that we need to set the root_gb to 0 or not? I don't know.

But I find out that the root_gb will been added into local_gb_used of
compute_node so that it will impact the next scheduling. Think about a use
case, the local_gb of compute_node is 10, boot instance from volume with
the root_gb=5 flavor, in this case, I can only boot 2 boot-from-volume
instances on the compute_nodes, although these instances don't use the
local disk of compute_nodes.

I find a patch that try to fix this issue,

I want to know that which solution is better for you?

Solution #1: boot instance from volume with the root_gb=0 flavor.
Solution #2: add some special logic in order to correct the disk usage,
like patch #136284
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