Segments are never modified after they have been written, so the merged segment is written to a new file. The merging and compaction of segments can be done in a background thread, and while it is going on, we can still continue to serve read requests using the old segment files, and write requests to the latest segment file. After the merging process is complete, we switch read requests to using the new merged segment instead of the old segments—and then the old segment files can simply be deleted.
