Thanks for your comments, Sake and David. Depending on the customer's budget we'll then either run some tests with the documented stretch mode or build our own stretch mode in "legacy" style by creating a suitable crush rule. Thanks! Eugen Zitat von Sake Ceph <ceph@paulusma.eu>:

I believe they are working on it or want to work on it to revert from a stretched cluster, because of the reason you mention: if the other datacenter is totally burned down, you maybe want for the time being switch to one datacenter setup.

Best regards, Sake

...

Op 09-11-2023 11:18 CET schreef Eugen Block <eblock@nde.ag>:

Hi,

I'd like to ask for confirmation how I understand the docs on stretch mode [1]. It requires exact size 4 for the rule? Other sizes are not supported/won't work, for example size 6? Are there clusters out there which use this stretch mode? Once stretch mode is enabled, it's not possible to get out of it. How would one deal with a burnt down datacenter which can take months to rebuild? In a "self-managed" stretch cluster (let's say size 6) I could simply change the crush rule to not consider the failed datacenter anymore, deploy an additional mon somewhere and maybe reduce the size/min_size. Am I missing something?

Thanks, Eugen

[1] https://docs.ceph.com/en/reef/rados/operations/stretch-mode/#id2

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No it’s not too late, it will take some time till we get there. So thanks for the additional input, I am aware of the MON communication. Zitat von Sake Ceph <ceph@paulusma.eu>:

Don't forget with stretch mode, osds only communicate with mons in the same DC and the tiebreaker only communicate with the other mons (to prevent split brain scenarios).

Little late response, but I wanted you to know this :)