In a leveled column family, there can be many thousands of sstables, since
each sstable is limited to a relatively small size (160M by default).
With the current approach of reading from all sstables in parallel, cpu
quickly becomes a bottleneck as we need to check the bloom filter for each
of these sstables.
This patch addresses the problem by introducing a
compaction-strategy-specific data structure for holding sstables. This
data structure has a method to obtain the sstables used for a read.
For leveled compaction strategy, this data structure is an interval map,
which can be efficiently used to select the right sstables.
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