Hi, Suggestion first: ceph.com site could have some best practice rules for adding new OSDs. Googling regarding this topic reveals that people have questions like: - may I add serveral OSDSs at once? - may I completely change crushmap online so that pgs get completely relocated? - what config parameters help to reduce backfill load? Until that still have this theoretical question on CRUSH algorithm. We have ceph cluster with 5 osd hosts, CRUSH rule orders ceph to put replicas one copy per host. If we add 2 osds simultaneously in different hosts - how CRUSH guarantees that some existing pg that now should be located on those new 2 osds does not get unavailable? Should be something with epochs I suppose? Have found thread mentioning that people have tested completely remapping pgs http://lists.ceph.com/pipermail/ceph-users-ceph.com/2017-July/019577.html Still it is not clear what are the theoretical constraints on adding bunches of OSDs (except backfill load). For example if pg gets relocated several times in a row(in case osds get added not waiting degradation to resolve) - how long that chain of previously allocated pgs can be? Any comments demistifying this are appreciated. Best regards Ugis -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
