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b6a7023f68
This is a step towards more flexibility in managing tablets. A prerequisite before we can split individual tablets, isolating hot partitions, and evening-out tablet sizes by shifting boundaries. After this patch, the system can handle tables with arbitrary tablet count. Tablet allocator is still rounding up desired tablet count to the nearest power of two when allocating tablets for a new table, so unless the tablet map is allocated in some other way, the counts will be still a power of two. We plan to utilize arbitrary count when migrating from vnodes to tablets, by creating a tablet map which matches vnode boundaries. One of the reasons we don't give up on power-of-two by default yet is that it creates an issue with merges. If tablet count is odd, one of the tablets doesn't have a sibling and will not be merged. That can obviously cause imbalance of token space and tablet sizes between tablets. To limit the impact, this patch dynamically chooses which tablet to isolate when initiating a merge. The largest tablet is chosen, as that will minimize imbalance. Otherwise, if we always chose the last tablet to isolate, its size would remain the same while other tablets double in size with each odd-count merge, leading to imbalance. The imbalance will still be there, but the difference in tablet sizes is limited to 2x. Example (3 tablets): [0] owns 1/3 of tokens [1] owns 1/3 of tokens [2] owns 1/3 of tokens After merge: [0] owns 2/3 of tokens [1] owns 1/3 of tokens What we would like instead: Step 1 (split [1]): [0] owns 1/3 of tokens [1] old 1.left, owns 1/6 of tokens [2] old 1.right, owns 1/6 of tokens [3] owns 1/3 of tokens Step 2 (merge): [0] owns 1/2 of tokens [1] owns 1/2 of tokens To do that, we need to be able to split individual tablets, but we're not there yet.