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On 10/28/2014 12:47 PM, Duncan Thomas wrote:
> 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?
Changing Nova disk names is a loooong shot. It's likely I will be doing 
something else by the time that gets merged:) So we are left with the 
two options of 1) using a shared volume group and, thus, complicating 
life for Cinder or 2) using separate volume groups potentially causing 
headaches for DevStack. I am trying to figure out which of these two is 
the lesser evil. It seems that Dean's concerns can be addressed, though, 
he still has to weigh in on the proposed mitigation approaches. I have 
little understanding of what problems a shared Cinder-Nova volume group 
would cause for Cinder testing. How hard would it be to make the tests 
work with a shared volume group?

Dan
> On 28 October 2014 14:27, Dan Genin <daniel.genin at 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.genin at 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.
>>>
>>>
>>> _______________________________________________
>>> OpenStack-dev mailing list
>>> OpenStack-dev at lists.openstack.org
>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>>>
>>
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