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Le 23/09/2015 21:45, John Griffith a écrit :
>
>
> On Wed, Sep 23, 2015 at 1:34 PM, Matt Riedemann 
> <mriedem at linux.vnet.ibm.com <mailto:mriedem at linux.vnet.ibm.com>> wrote:
>
>
>
>     On 9/23/2015 2:15 PM, Matt Riedemann wrote:
>
>
>
>         On 9/23/2015 1:46 PM, Ivan Kolodyazhny wrote:
>
>             Hi Matt,
>
>             In Liberty, we introduced allow_availability_zone_fallback
>             [1] option in
>             Cinder config as fix for bug [2]. If you set this option,
>             Cinder will
>             create volume in a default AZ instead of set volume into
>             the error state
>
>             [1]
>             https://github.com/openstack/cinder/commit/b85d2812a8256ff82934d150dbc4909e041d8b31
>
>             [2] https://bugs.launchpad.net/cinder/+bug/1489575
>
>             Regards,
>             Ivan Kolodyazhny
>
>             On Wed, Sep 23, 2015 at 9:00 PM, Matt Riedemann
>             <mriedem at linux.vnet.ibm.com
>             <mailto:mriedem at linux.vnet.ibm.com>
>             <mailto:mriedem at linux.vnet.ibm.com
>             <mailto:mriedem at linux.vnet.ibm.com>>> wrote:
>
>                 I came across bug 1496235 [1] today.  In this case the
>             user is
>                 booting an instance from a volume using source=image,
>             so nova
>                 actually does the volume create call to the volume
>             API.  They are
>                 booting the instance into a valid nova availability
>             zone, but that
>                 same AZ isn't defined in Cinder, so the volume create
>             request fails
>                 (since nova passes the instance AZ to cinder [2]).
>
>                 I marked this as invalid given how the code works.
>
>                 I'm posting here since I'm wondering if there are
>             alternatives worth
>                 pursuing.  For example, nova could get the list of AZs
>             from the
>                 volume API and if the nova AZ isn't in that list,
>             don't provide it
>                 on the volume create request.  That's essentially the
>             same as first
>                 creating the volume outside of nova and not specifying
>             an AZ, then
>                 when doing the boot from volume, provide the volume_id
>             as the source.
>
>                 The question is, is it worth doing that?  I'm not
>             familiar enough
>                 with how availability zones are meant to work between
>             nova and
>                 cinder so it's hard for me to have much of an opinion
>             here.
>
>                 [1] https://bugs.launchpad.net/nova/+bug/1496235
>                 [2]
>
>             https://github.com/openstack/nova/blob/master/nova/virt/block_device.py#L381-L383
>
>
>                 --
>
>                 Thanks,
>
>                 Matt Riedemann
>
>
>
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>
>         Sorry but that seems like a hack.
>
>         I'm trying to figure out the relationship between AZs in nova
>         and cinder
>         and so far no one seems to really know.  In the cinder IRC
>         channel I was
>         told there isn't one, which would mean we shouldn't even try
>         creating
>         the volume using the server instance AZ.
>
>         Also, if there is no relationship, I was trying to figure out
>         why there
>         is the cinder.cross_az_attach config option.  That was added
>         in grizzly
>         [1].  I was thinking maybe it was a legacy artifact from
>         nova-volume,
>         but that was dropped in grizzly.
>
>         So is cinder.cross_az_attach even useful?
>
>         [1] https://review.openstack.org/#/c/21672/
>
>
>     The plot thickens.
>
>     I was checking to see what change was made to start passing the
>     server instance az on the volume create call during boot from
>     volume, and that was [1] which was added in kilo to fix a bug
>     where boot from volume into a nova az will fail if
>     cinder.cross_az_attach=False and storage_availability_zone is set
>     in cinder.conf.
>
>     So I guess we can't just stop passing the instance az to the
>     volume create call.
>
>     But what I'd really like to know is how this is all used between
>     cinder and nova, or was this all some work done as part of a
>     larger effort that was never completed? Basically, can we
>     deprecate the cinder.cross_az_attach config option in nova and
>     start decoupling this code?
>
>     [1] https://review.openstack.org/#/c/157041/
>
>
>     -- 
>
>     Thanks,
>
>     Matt Riedemann
>
>
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>
> ​To be honest this is probably my fault, AZ's were pulled in as part 
> of the nova-volume migration to Cinder and just sort of died. Quite 
> frankly I wasn't sure "what" to do with them but brought over the 
> concept and the zones that existing in Nova-Volume.  It's been an 
> issue since day 1 of Cinder, and as you note there are little hacks 
> here and there over the years to do different things.
>
> I think your question about whether they should be there at all or not 
> is a good one.  We have had some interest from folks lately that want 
> to couple Nova and Cinder AZ's (I'm really not sure of any details or 
> use-cases here).
>
> My opinion would be until somebody proposes a clear use case and need 
> that actually works that we consider deprecating it.
>

Given what are currently AZs in Nova (explicit user-visible aggregates 
of compute nodes, see [1]), I tend to say that there is no reason to 
provide the AZ information to Cinder because AZs are not failure 
domains, just very specific placement information used for scheduling 
instances.

Based on some conversation we had on IRC, MHO is that we should at least 
deprecate the config flag that Matt discussed above and we should also 
plan to remove the AZ information from the Cinder call we do in Nova 
when creating a volume.

I leave Cinder community to discuss the plans about having AZs or not, 
but I also think that any reboot of the idea could necessarly need a 
renaming and no longer use the "availability zone" wording.


[1] 
http://docs.openstack.org/developer/nova/aggregates.html#availability-zones-azs

> While we're on the subject (kinda) I've never been a very fond of 
> having Nova create the volume during boot process either; there's a 
> number of things that go wrong here (timeouts almost guaranteed for a 
> "real" image) and some things that are missing last I looked like type 
> selection etc.
>
> We do have a proposal to talk about this at the Summit, so maybe we'll 
> have a descent primer before we get there :)
>
> Thanks,
>
> John
>
>
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