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TWCS can reshape at most 32 sstables spanning multiple windows, in a single compaction round. Which sstables are compacted together, when there are more than 32 sstables, is random. If sstables with overlapping windows are compacted together, then write amplification can be reduced because we may be able to push all the data to a window W in a single compaction round, so we'll not have to perform another compaction round later in W, to reduce its number of files. This is also very good to reduce the amount of transient file descriptors opened, because TWCS reshape first reshapes all sstables spanning multiple windows, so if all windows temporarily grow large in number of files, then there's a risk which file descriptors can be exhausted. Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com> Reviewed-by: Benny Halevy <bhalevy@scylladb.com> Message-Id: <20211013203046.233540-3-raphaelsc@scylladb.com>