doc/rados: explain what a device class breaks in stretch mode
The warning said device classes are "not supported" and that such a rule
"will not work", without saying what actually happens. Ceph accepts the
rule, honors the class, and places the data correctly. Peering is what
fails. The rule takes from a shadow tree, so an OSD's datacenter resolves
to "zone1~ssd", while the Monitors record the surviving zone as "zone1".
The two never match. Nothing looks wrong while both data centers are up,
but once one is lost, the PGs left behind cannot peer. Bringing the data
center back does not help either, because the cluster leaves degraded
stretch mode only once no PG is inactive.
Describe that, and point at the class-free rule under "Entering Stretch
Mode". Also fix the example, which wrote "type replicated class ssd". The
CRUSH grammar takes a device class on the take step.
projects / ceph.git / commit