On 7/16/2015 11:47 AM, Nikola Đipanov wrote:
On 07/16/2015 11:24 AM, Sean Dague wrote:
On 07/15/2015 01:41 PM, Andrew Laski wrote:
On 07/15/15 at 12:19pm, Matt Riedemann wrote:
<snip>
The other part of the discussion is around the API changes, not just
for libvirt, but having a microversion that removes the device from
the request so it's no longer optional and doesn't provide some false
sense that it works properly all of the time. We talked about this in
the nova channel yesterday and I think the thinking was we wanted to
get agreement on dropping that with a microversion before moving
forward with the libvirt change you have to ignore the requested
device name.
From what I recall, this was supposed to really only work reliably for
xen but now it actually might not, and would need to be tested again.
Seems we could start by checking the xen CI to see if it is running
the test_minimum_basic scenario test or anything in
test_attach_volume.py in Tempest.
This doesn't really work reliably for xen either, depending on what is
being done. For the xenapi driver Nova converts the device name
provided into an integer based on the trailing letter, so 'vde' becomes
4, and asks xen to mount the device based on that int. Xen does honor
that integer request so you'll get an 'e' device, but you could be
asking for hde and get an xvde or vice versa.
So this sounds like it's basically not working today. For Linux guests
it really can't work without custom in guest code anyway, given how
device enumeration works.
That feels to me like we remove it from the API with a microversion, and
when we do that just comment that trying to use this before that
microversion is highly unreliable (possibly dangerous) and may just
cause tears.
The problem with outright banning it is that we still have to support
people who want to use the older version meaning all of the code would
have to support it indefinitely (3.0 is not even on the horizon), given
the shady gains, I can't help but feel that this is needless complexity.
Huh? That's what the microversion in the v2.1 API is for - we add a
microversion that drops support for the device name in the API request,
if you're using a version of the API before that we log a warning that
it's unreliable and probably shouldn't be used. With the microversion
you're opting in to using it.
Also, not being able to specify device names would make it impossible to
implement certain features that EC2 API can provide, such as overriding
the image block devices without significant effort.
Huh? (x2) With your change you're ignoring the requested device name
anyway, so how does this matter? Also, the ec2 API is moving out of
tree so do we care what that means for the openstack compute API?
...
On a slight tangent, probably a better way to provide mount stability to
the guest is with FS labels. libvirt is already labeling the filesystems
it creates, and xenserver probably could as well. The infra folks ran
into an issue yesterday
http://status.openstack.org//elastic-recheck/#1475012 where using that
info was their fix.
I think the reason device_names are exposed in the API is that that was
the quickest way to provide a sort of an ID of a block device attached
to a certain instance that further API calls can then act upon.
It's not the same thing as deterministic devices, but deterministic
devices really aren't a thing on first boot unless you have guest agent
code, or only boot with one disk and hot plug the rest carefully.
Neither are really fun answers.
-Sean
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--
Thanks,
Matt Riedemann
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