Tomasz Grabiec f1bda8d4c1 tablets: load_balancer: Scale down tablet count to respect per-shard tablet count goal ...

The limit is enforced by controlling average per-shard tablet replica
count in a given DC, which is controlled by per-table tablet
count. This is effective in respecting the limit on individual shards
as long as tablet replicas are distributed evenly between shards.

There is no attempt to move tablets around in order to enforce limits
on individual shards in case of imbalance between shards.

If the average per-shard tablet count exceeds the limit, all tables
which contribute to it (have replicas in the DC) are scaled down
by the same factor. Due to rounding up to the nearest power of 2,
we may overshoot the per-shard goal by at most a factor of 2.

If different DCs want different scale factors of a given table, the
lowest scale factor is chosen for a given table.

The limit is configurable. It's a global per-cluster config which
controls how many tablet replicas per shard in total we consider to be
still ok. It controls tablet allocator behavior, when choosing initial
tablet count. Even though it's a per-node config, we don't support
different limits per node. All nodes must have the same value of that
config. It's similar in that regard to other scheduler config items
like tablets_initial_scale_factor and target_tablet_size_in_bytes.

2025-02-19 16:29:07 +01:00

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