Cassandra sstable filename format

Does anyone know what the character/letter "f" in the filename stands for? Is it arbitrary or does it have some meaning

test-f-9-Data.db
test-f-9-Filter.db
test-f-9-Index.db
test-f-9-Statis开发者_运维问答tics.db

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It's a version number - version f is the current SSTable format, I think.

See http://www.mail-archive.com/commits@cassandra.apache.org/msg15060.html