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>
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