[openstack-dev] [nova] nova cellsv2 and DBs / down cells / quotas

Dan Smith dms at danplanet.com
Thu Oct 25 14:42:37 UTC 2018

> I guess our architecture is pretty unique in a way but I wonder if
> other people are also a little scared about the whole all DB servers
> need to up to serve API requests?

When we started down this path, we acknowledged that this would create a
different access pattern which would require ops to treat the cell
databases differently. The input we were getting at the time was that
the benefits outweighed the costs here, and that we'd work on caching to
deal with performance issues if/when that became necessary.

> I’ve been thinking of some hybrid cellsv1/v2 thing where we’d still
> have the top level api cell DB but the API would only ever read from
> it. Nova-api would only write to the compute cell DBs.
> Then keep the nova-cells processes just doing instance_update_at_top to keep the nova-cell-api db up to date.

I'm definitely not in favor of doing more replication in python to
address this. What was there in cellsv1 was lossy, even for the subset
of things it actually supported (which didn't cover all nova features at
the time and hasn't kept pace with features added since, obviously).

About a year ago, I proposed that we add another "read only mirror"
field to the cell mapping, which nova would use if and only if the
primary cell database wasn't reachable, and only for read
operations. The ops, if they wanted to use this, would configure plain
old one-way mysql replication of the cell databases to a
highly-available server (probably wherever the api_db is) and nova could
use that as a read-only cache for things like listing instances and
calculating quotas. The reaction was (very surprisingly to me) negative
to this option. It seems very low-effort, high-gain, and proper re-use
of existing technologies to me, without us having to replicate a
replication engine (hah) in python. So, I'm curious: does that sound
more palatable to you?


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