bytes_on_disk is the sum of all sstable components.
As read_simple() fetches the file size before parsing the component,
bytes_on_disk can be added incrementally rather than an additional
step after all components were already parsed.
Likewise, write_simple() tracks the offset for each new component,
and therefore bytes_on_disk can also be added incrementally.
This simplifies s3 life as it no longer have to care about feeding
a bytes_on_disk, which is currently limited to data and index
sizes only.
Refs #13649.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
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