This is the last missing tracker among the major strategies. After
this, only DTCS is left.
To calculate the backlog, we will define the point of zero-backlog
as having all data in the last level. The backlog is then:
Sum(L in levels) sizeof(L) * (max_levels - L) * fan_out,
where:
* the fan_out is the amount of SSTables we usually compact with the
next level (usually 10).
* max_levels is the number of levels currently populated
* sizeof(L) is the total amount of data in a particular level.
Care is taken for the backlog not to jump when a new level has been just
recently created.
Aside from that, SSTables that accumulate in L0 can be subject to STCS.
We will then add a STCS backlog in those SSTables to represent that.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
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