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There are several reasons we want to do that. One is that it will give us more flexibility in distributing the load. We can subdivide tablets at any token, and achieve more evenly-sized tablets. In particular, we can isolate large partitions into separate tablets. We can also split and merge incrementally individual tablets. Currently, we do it for the whole table or nothing, which makes splits and merges take longer and cause wide swings of the count. This is not implemented in this PR yet, we still split/merge the whole table. Another reason is vnode to tablets migration. We now could construct a tablet map which matches exactly the vnode boundaries, so migration can happen transparently from CQL-coordinator point of view. Tablet count is still a power-of-two by default for newly created tables. It may be different if tablet map is created by non-standard means, or if per-table tablet option "pow2_count" is set to "false". build/release/scylla perf-tablets: Memory footprint for 131k tablets increased from 56 MiB to 58.1 MiB (+3.5%) Before: ``` Generating tablet metadata Total tablet count: 131072 Size of tablet_metadata in memory: 57456 KiB Copied in 0.014346 [ms] Cleared in 0.002698 [ms] Saved in 1234.685303 [ms] Read in 445.577881 [ms] Read mutations in 299.596313 [ms] 128 mutations Read required hosts in 247.482742 [ms] Size of canonical mutations: 33.945053 [MiB] Disk space used by system.tablets: 1.456761 [MiB] Tablet metadata reload: full 407.69ms partial 2.65ms ``` After: ``` Generating tablet metadata Total tablet count: 131072 Size of tablet_metadata in memory: 59504 KiB Copied in 0.032475 [ms] Cleared in 0.002965 [ms] Saved in 1093.877441 [ms] Read in 387.027100 [ms] Read mutations in 255.752121 [ms] 128 mutations Read required hosts in 211.202805 [ms] Size of canonical mutations: 33.954453 [MiB] Disk space used by system.tablets: 1.450162 [MiB] Tablet metadata reload: full 354.50ms partial 2.19ms ``` Closes scylladb/scylladb#28459 * github.com:scylladb/scylladb: test: boost: tablets: Add test for merge with arbitrary tablet count tablets, database: Advertise 'arbitrary' layout in snapshot manifest tablets: Introduce pow2_count per-table tablet option tablets: Prepare for non-power-of-two tablet count tablets: Implement merged tablet_map constructor on top of for_each_sibling_tablets() tablets: Prepare resize_decision to hold data in decisions tablets: table: Make storage_group handle arbitrary merge boundaries tablets: Make stats update post-merge work with arbitrary merge boundaries locator: tablets: Support arbitrary tablet boundaries locator: tablets: Introduce tablet_map::get_split_token() dht: Introduce get_uniform_tokens()