Re: [openstack-dev] [devstack] Enable LVM ephemeral storage for Nova

[Dan Genin]

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.

Thank you,
Dan

1. https://wiki.archlinux.org/index.php/LVM#LVM_Building_Blocks


On 10/21/2014 03:10 PM, Duncan Thomas wrote:

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?

Duncan Thomas

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. On Oct 21, 2014 4:07 PM, "Dan Genin" <daniel.ge...@jhuapl.edu <mailto: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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