Hey folks,
One of the challenges we have faced with the ability to attach a
single volume to multiple instances, is how to correctly detach that
volume. The issue is a bit complex, but I'll try and explain the
problem, and then describe one approach to solving one part of the
detach puzzle.
Problem:
When a volume is attached to multiple instances on the same host.
There are 2 scenarios here.
1) Some Cinder drivers export a new target for every attachment on a
compute host. This means that you will get a new unique volume path on
a host, which is then handed off to the VM instance.
2) Other Cinder drivers export a single target for all instances on a
compute host. This means that every instance on a single host, will
reuse the same host volume path.
When a user issues a request to detach a volume, the workflow boils down
to first calling os-brick's connector.disconnect_volume before calling
Cinder's terminate_connection and detach. disconnect_volume's job is to
remove the local volume from the host OS and close any sessions.
There is no problem under scenario 1. Each disconnect_volume only
affects the attached volume in question and doesn't affect any other VM
using that same volume, because they are using a different path that has
shown up on the host. It's a different target exported from the Cinder
backend/array.
The problem comes under scenario 2, where that single volume is shared
for every instance on the same compute host. Nova needs to be careful
and not call disconnect_volume if it's a shared volume, otherwise the
first disconnect_volume call will nuke every instance's access to that
volume.
Proposed solution:
Nova needs to determine if the volume that's being detached is a
shared or non shared volume. Here is one way to determine that.
Every Cinder volume has a list of it's attachments. In those
attachments it contains the instance_uuid that the volume is attached
to. I presume Nova can find which of the volume attachments are on the
same host. Then Nova can call Cinder's initialize_connection for each
of those attachments to get the target's connection_info dictionary.
This connection_info dictionary describes how to connect to the target
on the cinder backend. If the target is shared, then each of the
connection_info dicts for each attachment on that host will be
identical. Then Nova would know that it's a shared target, and then
only call os-brick's disconnect_volume, if it's the last attachment on
that host. I think at most 2 calls to cinder's initialize_connection
would suffice to determine if the volume is a shared target. This would
only need to be done if the volume is multi-attach capable and if there
are more than 1 attachments on the same host, where the detach is happening.
Walt
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