> From: Warren Wang <war...@wangspeed.com>
> Sent: Wednesday, October 12, 2016 10:02 PM
> To: Adam Kijak
> Cc: Abel Lopez; openstack-operators
> Subject: Re: [Openstack-operators] [openstack-operators][ceph][nova] How do 
> you handle Nova on Ceph?
> If fault domain is a concern, you can always split the cloud up into 3 
> regions, each having a dedicate Ceph cluster. It isn't necessarily going to 
> mean more hardware, just logical splits. This is kind of assuming that the 
> network doesn't share the same fault domain though.

This is not an option because having Region1-1, Region1-2, ..., Region1-10 
would not be very convenient for users.

> Alternatively, you can split the hardware for the Ceph boxes into multiple 
> clusters, and use multi backend Cinder to talk to the same set of hypervisors 
> to use multiple Ceph clusters. We're doing that to migrate from one Ceph 
> cluster to another. You can even mount a volume from each cluster into a 
> single instance.

Multiple Ceph clusters on Cinder is not a problem, I agree.
Unfortunately we use Ceph for Nova (disks of instances are on Ceph directly).

> Keep in mind that you don't really want to shrink a Ceph cluster too much. 
> What's "too big"? You should keep growing so that the fault domains aren't 
> too small (3 physical rack min), or you guarantee that the entire cluster 
> stops if you lose network.
> Just my 2 cents,

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