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ca284174d0
This patch adds infrastucture in various points in the system to allow us to determine the amount of work present as backlog from compactions. What needs to be done can be explained in three major pieces: 1) Add hooks in the points where sstables are added or inserted to a column family (or more precisely, to a compaction_strategy object). 2) Add hooks in reads and write monitors that allows a compaction backlog estimator (tracker) to become aware of bytes that are partially written and compacted away. 3) Add a per-column family class (compaction_backlog_tracker) that can be used to track work that is done and relevant to compactions (like the two above), and a compaction manager to provide a system-wide backlog based on the response of the individual trackers. The definition of how much backlog one has is strategy-specific. The Null strategy is easy, as it never really has any backlog, and so is the major strategy - since what it really matters is the backlog of the underlying compaction strategy. Although backlogs are strategy-specific, they should be "compatible", in the sense that if a particular strategy has more work to do, it should yield a higher number than its counterparts. All the others are presented in this patch as unimplemented: they will always advertise a mild backlog that should yield a constant CPU-utilization if used alone. Signed-off-by: Glauber Costa <glauber@scylladb.com>