Le 24/09/2015 09:04, Duncan Thomas a écrit :
Hi
I thought I was late on this thread, but looking at the time stamps,
it is just something that escalated very quickly. I am honestly
surprised an cross-project interaction option went from 'we don't seem
to understand this' to 'deprecation merged' in 4 hours, with only a 12
hour discussion on the mailing list, right at the end of a cycle when
we're supposed to be stabilising features.
So, I agree it was maybe a bit too quick hence the revert. That said,
Nova master is now Mitaka, which means that the deprecation change was
provided for the next cycle, not the one currently stabilising.
Anyway, I'm really all up with discussing why Cinder needs to know the
Nova AZs.
I proposed a session at the Tokyo summit for a discussion of Cinder
AZs, since there was clear confusion about what they are intended for
and how they should be configured.
Cool, count me in from the Nova standpoint.
Since then I've reached out to and gotten good feedback from, a number
of operators. There are two distinct configurations for AZ behaviour
in cinder, and both sort-of worked until very recently.
1) No AZs in cinder
This is the config where a single 'blob' of storage (most of the
operators who responded so far are using Ceph, though that isn't
required). The storage takes care of availability concerns, and any AZ
info from nova should just be ignored.
2) Cinder AZs map to Nova AZs
In this case, some combination of storage / networking / etc couples
storage to nova AZs. It is may be that an AZ is used as a unit of
scaling, or it could be a real storage failure domain. Eitehr way,
there are a number of operators who have this configuration and want
to keep it. Storage can certainly have a failure domain, and limiting
the scalability problem of storage to a single cmpute AZ can have
definite advantages in failure scenarios. These people do not want
cross-az attach.
Ahem, Nova AZs are not failure domains - I mean the current
implementation, in the sense of many people understand what is a failure
domain, ie. a physical unit of machines (a bay, a room, a floor, a
datacenter).
All the AZs in Nova share the same controlplane with the same message
queue and database, which means that one failure can be propagated to
the other AZ.
To be honest, there is one very specific usecase where AZs *are* failure
domains : when cells exact match with AZs (ie. one AZ grouping all the
hosts behind one cell). That's the very specific usecase that Sam is
mentioning in his email, and I certainly understand we need to keep that.
What are AZs in Nova is pretty well explained in a quite old blogpost :
http://blog.russellbryant.net/2013/05/21/availability-zones-and-host-aggregates-in-openstack-compute-nova/
We also added a few comments in our developer doc here
http://docs.openstack.org/developer/nova/aggregates.html#availability-zones-azs
tl;dr: AZs are aggregate metadata that makes those aggregates of compute
nodes visible to the users. Nothing more than that, no magic sauce.
That's just a logical abstraction that can be mapping your physical
deployment, but like I said, which would share the same bus and DB.
Of course, you could still provide networks distinct between AZs but
that just gives you the L2 isolation, not the real failure domain in a
Business Continuity Plan way.
What puzzles me is how Cinder is managing a datacenter-level of
isolation given there is no cells concept AFAIK. I assume that
cinder-volumes are belonging to a specific datacenter but how is managed
the controlplane of it ? I can certainly understand the need of affinity
placement between physical units, but I'm missing that piece, and
consequently I wonder why Nova need to provide AZs to Cinder on a
general case.
My hope at the summit session was to agree these two configurations,
discuss any scenarios not covered by these two configuration, and nail
down the changes we need to get these to work properly. There's
definitely been interest and activity in the operator community in
making nova and cinder AZs interact, and every desired interaction
I've gotten details about so far matches one of the above models.
I'm all with you about providing a way for users to get volume affinity
for Nova. That's a long story I'm trying to consider and we are
constantly trying to improve the nova scheduler interfaces so that other
projects could provide resources to the nova scheduler for decision
making. I just want to consider whether AZs are the best concept for
that or we should do thing by other ways (again, because AZs are not
what people expect).
Again, count me in for the Cinder session, and just lemme know when the
session is planned so I could attend it.
-Sylvain
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