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91bbf90f71
The btree_merge code was pinning leaf blocks for all input btrees as it iterated over them. This doesn't work when there are a very large number of input btrees. It can run out of memory trying to hold a reference to a 64KiB leaf block for each input root. This reworks the btree merging code. It reads a window of blocks from all input trees to get a set of merged items. It can take multiple passes to complete the merge but by setting the merge window large enough this overhead is reduced. Merging now consumes a fixed amount of memory rather than using memory proportional to the number of input btrees. Signed-off-by: Zach Brown <zab@versity.com>