This series achieves two things:
1) changes default number of tablet replicas per shard to be 10 in order to reduce load imbalance between shards
This will result in new tables having at least 10 tablet replicas per
shard by default.
We want this to reduce tablet load imbalance due to differences in
tablet count per shard, where some shards have 1 tablet and some
shards have 2 tablets. With higher tablet count per shard, this
difference-by-one is less relevant.
Fixes https://github.com/scylladb/scylladb/issues/21967
2) introduces a global goal for tablet replica count per shard and adds logic to tablet scheduler to respect it by controlling per-table tablet count
The per-shard goal 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.
The scaling is applied after computing desired tablet count due to
all other factors: per-table tablet count hints, defaults, average tablet size.
If different DCs want different scale factors of a given table, the
lowest scale factor is chosen for a given table.
When creating a new table, its tablet count is determined by tablet
scheduler using the scheduler logic, as if the table was already created.
So any scaling due to per-shard tablet count goal is reflected immediately
when creating a table. It may however still take some time for the system
to shrink existing tables. We don't reject requests to create new tables.
Fixes#21458Closesscylladb/scylladb#22522
* github.com:scylladb/scylladb:
config, tablets: Allow tablets_initial_scale_factor to be a fraction
test: tablets_test: Test scaling when creating lots of tables
test: tablets_test: Test tablet count changes on per-table option and config changes
test: tablets_test: Add support for auto-split mode
test: cql_test_env: Expose db config
config: Make tablets_initial_scale_factor live-updateable
tablets: load_balancer: Pick initial_scale_factor from config
tablets, load_balancer: Fix and improve logging of resize decisions
tablets, load_balancer: Log reason for target tablet count
tablets: load_balancer: Move hints processing to tablet scheduler
tablets: load_balancer: Scale down tablet count to respect per-shard tablet count goal
tablets: Use scheduler's make_sizing_plan() to decide about tablet count of a new table
tablets: load_balancer: Determine desired count from size separately from count from options
tablets: load_balancer: Determine resize decision from target tablet count
tablets: load_balancer: Allow splits even if table stats not available
tablets: load_balancer: Extract make_sizing_plan()
tablets: Add formatter for resize_decision::way_type
tablets: load_balancer: Simplify resize_urgency_cmp()
tablets: load_balancer: Keep config items as instance members
locator: network_topology_strategy: Simplify calculate_initial_tablets_from_topology()
tablets: Change the meaning of initial_scale to mean min-avg-tablets-per-shard
tablets: Set default initial tablet count scale to 10
tablets: network_topology_stragy: Coroutinize calculate_initial_tablets_from_topology()
tablets: load_balancer: Extract get_schema_and_rs()
tablets: load_balancer: Drop test_mode
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