cthier at gmail
Aug 14, 2012, 10:00 PM
Post #8 of 13
Re: [openstack-dev] [nova] Disk attachment consistency
[In reply to]
First, thanks for bringing this up for discussion. Coincidentally a
similar discussion had come up with our teams, but I had pushed it
aside at the time due to time constraints. It is a tricky problem to
solve generally for all hypervisors. See my comments inline:
On Mon, Aug 13, 2012 at 10:35 PM, Vishvananda Ishaya
<vishvananda [at] gmail> wrote:
> Long term solutions
> We probably shouldn't expose a device path, it should be a device number. This is probably the right change long term, but short term we need to make the device name make sense somehow. I want to delay the long term until after the summit, and come up with something that works short-term with our existing parameters and usage.
I totally agree with delaying the long term discussion, and look
forward to discussing these types of issues more at the summit.
> The first proposal I have is to make the device parameter optional. The system will automatically generate a valid device name that will be accurate for xen and kvm with guest kernel 3.2, but will likely be wrong for old kvm guests in some situations. I think this is definitely an improvement and only a very minor change to an extension api (making a parameter optional, and returning the generated value of the parameter).
I could get behind this, and was was brought up by others in our group
as a more feasible short term solution. I have a couple of concerns
with this. It may cause just as much confusion if the api can't
reliably determine which device a volume is attached to. I'm also
curious as to how well this will work with Xen, and hope some of the
citrix folks will chime in. From an api standpoint, I think it would
be fine to make it optional, as any client that is using old api
contract will still work as intended.
> (review at https://review.openstack.org/#/c/10908/)
> The second proposal I have is to use a feature of kvm attach and set the device serial number. We can set it to the same value as the device parameter. This means that a device attached to /dev/vdb may not always be at /dev/vdb (with old kvm guests), but it will at least show up at /dev/disk/by-id/virtio-vdb consistently.
> (review coming soon)
> First question: should we return this magic path somewhere via the api? It would be pretty easy to have horizon generate it but it might be nice to have it show up. If we do return it, do we mangle the device to always show the consistent one, or do we return it as another parameter? guest_device perhaps?
> Second question: what should happen if someone specifies /dev/xvda against a kvm cloud or /dev/vda against a xen cloud?
> I see two options:
> a) automatically convert it to the right value and return it
I thought that it already did this, but I would have to go back and
double check. But it seemed like for xen at least, if you specify
/dev/vda, Nova would change it to /dev/xvda.
> b) fail with an error message
I don't have a strong opinion either way, as long as it is documented
correctly. I would suggest thought that if it has been converting it
in the past, that we continue to do so.
> Third question: what do we do if someone specifies a device value to a kvm cloud that we know will not work. For example the vm has /dev/vda and /dev/vdb and they request an attach at /dev/vdf. In this case we know that it will likely show up at /dev/vdc. I see a few options here and none of them are amazing:
> a) let the attach go through as is.
> advantages: it will allow scripts to work without having to manually find the next device.
> disadvantages: the device name will never be correct in the guest
> b) automatically modify the request to attach at /dev/vdc and return it
> advantages: the device name will be correct some of the time (kvm guests with newer kernels)
> disadvantages: sometimes the name is wrong anyway. The user may not expect the device number to change
> c) fail and say, the next disk must be attached at /dev/vdc:
> advantages: explicit
> disadvantages: painful, incompatible, and the place we say to attach may be incorrect anyway (kvm guests with old kernels)
I would choose b, as it tries to get things in the correct state. c
is a bad idea as it would change the overall api behavior, and current
clients wouldn't expect it.
There are also a couple of other interesting tidbits, that may be
related, or at least be worthwhile to know while discussing this.
Xen Server 6.0 has a limit of 16 virtual devices per guest instance.
Experimentally it also expects those to be /dev/xvda - /dev/xvdp. You
can't for example attach a device to /dev/xvdq, even if there are no
other devices attached to the instance. If you attempt to do this,
the volume will go in to the attaching state, fail to attach, and then
fall back to the available state (This can be a bit confusing to new
users who try to do so). Does anyone know if there are similar
limitations for KVM?
Also if you attempt to attach a volume to a deivce that already
exists, it will silently fail and go back to available as well. In
this new scheme should it fail like that, or should it attempt to
attach it to the next available device, or error out? Perhaps a
better question here, is for this initial consistency, is the goal to
try to be consistent just when there is no device sent, or to also be
consistent when the device is sent as well.
There was another idea, also brought up in our group. Would it be
possible to add a call that would return a list of available devices
to be attached to? In the case of Xen, it would return a list of
devices /dev/xvda-p that were not used. In the case of KVM, it would
just return the next available device name. At least in this case,
user interfaces and command line tools could use this to validate the
input the user provides (or auto generate the device to be used if the
user doesn't select a device).
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