Hi Dan

You're quite right, the nesting isn't as I thought it was, sorry to mislead you.

It isn't a show stopper, it just makes testing some proposed useful
functionality slightly harder. If nova were to namespace its volumes
(e.g. start all the volume names with nova-*) then that would allow
the problem to be easily worked around in the test, does that sound
reasonable?

On 28 October 2014 14:27, Dan Genin <daniel.ge...@jhuapl.edu> wrote:
> Duncan, I don't think it's possible to have multiple volume groups using the
> same physical volume[1]. In fact, counter-intuitively (at least to me) the
> nesting actually goes the other way with multiple physical volumes
> comprising a single volume group. The LVM naming scheme actually makes more
> sense with this hierarchy.
>
> So this brings us back to the original proposal of having separate backing
> files for Cinder and Nova which Dean thought might take too much space.
>
> Duncan, could you please elaborate on the pain a single volume group is
> likely to cause for Cinder? Is it a show stopper?
>
> Thank you,
> Dan
>
> 1. https://wiki.archlinux.org/index.php/LVM#LVM_Building_Blocks
>
>
> On 10/21/2014 03:10 PM, Duncan Thomas wrote:
>
> Sharing the vg with cinder is likely to cause some pain testing proposed
> features cinder reconciling backend with the cinder db. Creating a second vg
> sharing the same backend pv is easy and avoids all such problems.
>
> Duncan Thomas
>
> On Oct 21, 2014 4:07 PM, "Dan Genin" <daniel.ge...@jhuapl.edu> wrote:
>>
>> Hello,
>>
>> I would like to add to DevStack the ability to stand up Nova with LVM
>> ephemeral
>> storage. Below is a draft of the blueprint describing the proposed
>> feature.
>>
>> Suggestions on architecture, implementation and the blueprint in general
>> are very
>> welcome.
>>
>> Best,
>> Dan
>>
>> ========================
>> Enable LVM ephemeral storage for Nova
>> ========================
>>
>> Currently DevStack supports only file based ephemeral storage for Nova,
>> e.g.,
>> raw and qcow2. This is an obstacle to Tempest testing of Nova with LVM
>> ephemeral
>> storage, which in the past has been inadvertantly broken
>> (see for example, https://bugs.launchpad.net/nova/+bug/1373962), and to
>> Tempest
>> testing of new features based on LVM ephemeral storage, such as LVM
>> ephemeral
>> storage encryption.
>>
>> To enable Nova to come up with LVM ephemeral storage it must be provided a
>> volume group. Based on an initial discussion with Dean Troyer, this is
>> best
>> achieved by creating a single volume group for all services that
>> potentially
>> need LVM storage; at the moment these are Nova and Cinder.
>>
>> Implementation of this feature will:
>>
>>  * move code in lib/cinder/cinder_backends/lvm to lib/lvm with appropriate
>>    modifications
>>
>>  * rename the Cinder volume group to something generic, e.g., devstack-vg
>>
>>  * modify the Cinder initialization and cleanup code appropriately to use
>>    the new volume group
>>
>>  * initialize the volume group in stack.sh, shortly before services are
>>    launched
>>
>>  * cleanup the volume group in unstack.sh after the services have been
>>    shutdown
>>
>> The question of how large to make the common Nova-Cinder volume group in
>> order
>> to enable LVM ephemeral Tempest testing will have to be explored.
>> Although,
>> given the tiny instance disks used in Nova Tempest tests, the current
>> Cinder volume group size may already be adequate.
>>
>> No new configuration options will be necessary, assuming the volume group
>> size
>> will not be made configurable.
>>
>>
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>>
>
>
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-- 
Duncan Thomas

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