[openstack-dev] [Nova][Cinder] Multi-attach, determining when to call os-brick's connector.disconnect_volume

Ildikó Váncsa ildiko.vancsa at ericsson.com
Fri Feb 12 20:26:38 UTC 2016

Hi Walt,

Thanks for describing the bigger picture.

In my opinion when we will have microversion support available in Cinder that will give us a bit of a freedom and also possibility to handle these difficulties.

Regarding terminate_connection we will have issues with live_migration as it is today. We need to figure out what information would be best to feed back to Cinder from Nova, so we should figure out what API we would need after we are able to introduce it in a safe way. I still see benefit in storing the connection_info for the attachments.

Also I think the multiattach support should be disable for the problematic drivers like lvm, until we don't have a solution for proper detach on the whole call chain.

Best Regards,

> -----Original Message-----
> From: Walter A. Boring IV [mailto:walter.boring at hpe.com]
> Sent: February 11, 2016 18:31
> To: openstack-dev at lists.openstack.org
> Subject: Re: [openstack-dev] [Nova][Cinder] Multi-attach, determining when to call os-brick's connector.disconnect_volume
> There seems to be a few discussions going on here wrt to detaches.   One
> is what to do on the Nova side with calling os-brick's disconnect_volume, and also when to or not to call Cinder's
> terminate_connection and detach.
> My original post was simply to discuss a mechanism to try and figure out the first problem.  When should nova call brick to remove the
> local volume, prior to calling Cinder to do something.
> Nova needs to know if it's safe to call disconnect_volume or not. Cinder already tracks each attachment, and it can return the
> connection_info
> for each attachment with a call to initialize_connection.   If 2 of
> those connection_info dicts are the same, it's a shared volume/target.
> Don't call disconnect_volume if there are any more of those left.
> On the Cinder side of things, if terminate_connection, detach is called, the volume manager can find the list of attachments for a
> volume, and compare that to the attachments on a host.  The problem is, Cinder doesn't track the host along with the instance_uuid in
> the attachments table.  I plan on allowing that as an API change after microversions lands, so we know how many times a volume is
> attached/used on a particular host.  The driver can decide what to do with it at
> terminate_connection, detach time.     This helps account for
> the differences in each of the Cinder backends, which we will never get all aligned to the same model.  Each array/backend handles
> attachments different and only the driver knows if it's safe to remove the target or not, depending on how many attachments/usages
> it has
> on the host itself.   This is the same thing as a reference counter,
> which we don't need, because we have the count in the attachments table, once we allow setting the host and the instance_uuid at
> the same time.
> Walt
> > On Tue, Feb 09, 2016 at 11:49:33AM -0800, Walter A. Boring IV wrote:
> >> 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.
> >
> > This problem isn't actually new. It is a problem we already have in
> > Nova even with single attachments per volume.  eg, with NFS and SMBFS
> > there is a single mount setup on the host, which can serve up multiple volumes.
> > We have to avoid unmounting that until no VM is using any volume
> > provided by that mount point. Except we pretend the problem doesn't
> > exist and just try to unmount every single time a VM stops, and rely
> > on the kernel failing umout() with EBUSY.  Except this has a race
> > condition if one VM is stopping right as another VM is starting
> >
> > There is a patch up to try to solve this for SMBFS:
> >
> >     https://review.openstack.org/#/c/187619/
> >
> > but I don't really much like it, because it only solves it for one
> > driver.
> >
> > I think we need a general solution that solves the problem for all
> > cases, including multi-attach.
> >
> > AFAICT, the only real answer here is to have nova record more info
> > about volume attachments, so it can reliably decide when it is safe to
> > release a connection on the host.
> >
> >
> >> 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.
> > As above, we need to solve this more generally than just multi-attach,
> > even single-attach is flawed today.
> >
> > Regards,
> > Daniel
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