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Merge co-location can emit migrations across racks even when RF=#racks, reducing availability and affecting consistency of base-view pairing. Given replica set of sibling tablets T0 and T1 below: [T0: (rack1,rack3,rack2)] [T1: (rack2,rack1,rack3)] Merge will co-locate T1:rack2 into T0:rack1, T1 will be temporarily only at only a subset of racks, reducing availability. This is the main problem fixed by this patch. It also lays the ground for consistent base-view replica pairing, which is rack-based. For tables on which views can be created we plan to enforce the constraint that replicas don't move across racks and that all tablets use the same set of racks (RF=#racks). This patch avoids moving replicas across racks unless it's necessary, so if the constraint is satisfied before merge, there will be no co-locating migrations across racks. This constraint of RF=#racks is not enforced yet, it requires more extensive changes. Fixes #22994. Refs #17265. This patch is based on Raphael's work done in PR #23081. The main differences are: 1) Instead of sorting replicas by rack, we try to find replicas in sibling tablets which belong to the same rack. This is similar to how we match replicas within the same host. It reduces number of across-rack migrations even if RF!=#racks, which the original patch didn't handle. Unlike the original patch, it also avoids rack-overloaded in case RF!=#racks 2) We emit across-rack co-locating migrations if we have no other choice in order to finalize the merge This is ok, since views are not supported with tablets yet. Later, we will disallow this for tables which have views, and we will allow creating views in the first place only when no such migrations can happen (RF=#racks). 3) Added boost unit test which checks that rack overload is avoided during merge in case RF<#racks 4) Moved logging of across-rack migration to debug level 5) Exposed metric for across-rack co-locating migrations Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com> Signed-off-by: Tomasz Grabiec <tgrabiec@scylladb.com> Closes scylladb/scylladb#23247