The endpoint is the following:
POST /storage_service/vnode_tablet_migrations/keyspaces/{keyspace}
Its purpose is to start the migration of a whole keyspace from vnodes to
tablets.
When called, Scylla will synchronously create a tablet map for each
table in the specified keyspace. The tablet maps of all tables are
identical and they mirror the vnode layout; they contain one tablet per
vnode and each tablet uses the same replica hosts and token boundaries
as the corresponding vnode.
The only difference from vnodes lies in the sharding approach. Tablets
are assigned to a single shard - using a round-robin strategy in this
patch - whereas vnodes are distributed evenly across all shards. If the
tablet count per shard is low and tablet sizes are uneven, or some
shards have more tablets than others, performance may degrade during the
migration process. For example, a cluster with i8g.48xlarge (192 vCPUs),
256 vnodes per node and RF=3 will have 256 * 3 / 192 vCPUs = 4 tablet
replicas per shard during the migration. One additional tablet or a
double-sized tablet would cause 25% overcommit.
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>
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