Open Stack

> -----Original Message-----
> From: Matt Riedemann [mailto:mriedem at linux.vnet.ibm.com]
> Sent: 18 February 2016 03:11
> To: openstack at lists.openstack.org
> Subject: Re: [Openstack] [Nova] [Cinder] Block device name / ordering
> 
> 
> 
> On 2/17/2016 6:49 PM, Bruno L wrote:
> > Hi everyone,
> >
> > Could not figure this one out based on docs, wiki, or previous posts
> > on the mailing list.
> >
> > We use KVM as our hypervisor on the Catalyst Cloud in New Zealand.
> > Unlike Xen, KVM does not support setting the device name (eg:
> > /dev/vdc) of a cinder volume within the guest. If a user specifies the
> > device name it is ignored and cinder will display the device name that
> > has been assigned.
> >
> > To what extent is the device name (mountpoint) displayed by Cinder /
> > Nova reliable? Is Cinder just naming attachments in alphabetical order
> > hoping that it is correct, or actually obtaining the device name from
> > the hypervisor?
> >
> > If a user attaches 2 volumes with the same size to a compute instance
> > (eg: one for a database and one for media assets), how can he or she
> > identify which one is what? From what I can see on our compute
> > instances, the device ID or UUID does not match the volume name or
> > UUID displayed by OpenStack. What is the best way to ensure you are
> > dealing with the correct device?
> >

We label the volumes when formatting them with mkfs. The label can then be
used in fstab so the device is not important.

It is also useful when volumes get moved around between machines.

> > Do you know if there is work going on in KVM to allow a device name to
> > be specified like in Xen?
> >
> > Cheers,
> > Bruno
> >
> > Catalyst Cloud
> > http://www.catalyst.net.nz/catalyst-cloud
> >
> >
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> >
> 
> It's not actually cinder that's picking the device name (mountpoint), it's
the
> libvirt driver (more specifically, the nova.virt.libvirt.blockinfo code).
> 
> The change to ignore the user-supplied device name for the libvirt driver
> went into liberty [1]. I don't know all of the details behind it, but my
> understanding was requesting a specific device was not reliable with
libvirt,
> so it's best to let libvirt pick the mountpoint for you.
> ndipanov and/or danpb in the #openstack-nova freenode channel would be
> able to give more information.
> 
> As for figuring out which mountpoint the attachments are on, you have to
> wait for the volume status to go to 'in-use' and then from the volume GET
> you can get the attachment information (there should be a list of dicts in
the
> response that have the attachment IDs and mountpoints in them). I believe
> since we don't support multiattach for volumes yet in nova, there would
only
> be one attachment in the list.
> 
> As for figuring out what's what, if you are naming the volumes that you're
> attaching to something like 'db' and 'media', then you should know which
is
> which, right? Then just get the mountpoint from the volume attachment
> meta that's available via the Cinder REST API.
> 
> Hope this helps.
> 
> [1] https://review.openstack.org/#/c/189632/
> 
> --
> 
> Thanks,
> 
> Matt Riedemann
> 
> 
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