Compaction Byte Rate Metrics are Confusing

[tpetracca]

If you do a 10GB compaction over 10 minutes, you don't see 1/60th of a GB per second over 10 minutes like you'd expect. You see the 10GB as a single instantaneous event.

[leonz]

This is because of the way the compaction is marked - https://github.com/palantir/cassandra/pull/80/files

cfs.metric.compactionBytesWritten.inc(endsize);