Although replace_sstables is supposed to be called
only once per {old_ssts, new_ssts} it is safer
to update `_total_bytes` with `sst->data_size()`
only if the sst was inserted/erased successfully.
Otherwise _total_bytes may go out of sync with the
contents of _all.
That said, the next step should be to refer to the
compaction_group's main sstable set directly rather
than maintaining a "shadow" set in the tracker.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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