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,

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 <mailto:daniel.ge...@jhuapl.edu>> wrote:


    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

    Suggestions on architecture, implementation and the blueprint in
    general are very


    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
    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

     * rename the Cinder volume group to something generic, e.g.,

     * 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

     * cleanup the volume group in unstack.sh after the services have been

    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.
    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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