Ceph » RADOS

Here are some of the user's perspective on the feature:

Currently it is not possible to disable "stretch mode". Hereby I request to consider making an exit strategy for "stretch mode" a possibility. Use cases that might benefit from this:

- stretch cluster that is permanently degraded because of the permanent loss of a data center (or the specific bucket used)
- convert (expand) a "stretch mode" cluster to a traditional setup like a 3 data center setup

Stefan Kooman

Hi,

I have played with stretch clusters a bit but never managed to un-stretch them fully.

From my experience (using Pacific 16.2.9), once the stretch mode is enabled, the replicated pools switch to the stretch_rule with size 4, min_size 2, and require at least one replica in each datacenter. If one of the two DCs fails, you will still be able to change the crush rule of your pools back to the traditional replicated rule, and set size 3, min_size 2. After recovery, your objects won't be degraded anymore and stored 3 replicas in the surviving DC, but the cluster would still complain about one DC down (with related osds and mons).

It does not seem to exist a 'ceph mon disable_stretch_mode' and the monmaptool does not implement actions to disable stretch mode and remove the tiebreaker mon.

I'd be curious to know if it is possible to disable stretch_mode completely too.

Cheers,
Enrico