Glauber Costa c3f27784de database: guarantee a minimum amount of shares when manual operations are requested. ...

We have found issues when a flush is requested outside the usual
memtable flush loop and because there is not a lot of data the
controller will not have a high amount of shares.

To prevent this, this patch guarantees some minimum amount of shares
when extraneous operations (nodetool flush, commitlog-driven flush, etc)
are requested.

Another option would be to add shares instead of guarantee a minimum.
But in my view the approach I am taking here has two main advantages:

1) It won't cause spikes when those operations are requested
2) It is cumbersome to add shares in the current infrastructure, as just
adding backlog can cause shares to spike. Consider this example:

  Backlog is within the first range of very low backlog (~0.2). Shares
  for this would be around ~20. If we want to add 200 shares, that is
  equivalent to a backlog of 0.8. Once we add those two backlogs
  together, we end up with 1 (max backlog).

Fixes #3761

Tests: unit (release)

Signed-off-by: Glauber Costa <glauber@scylladb.com>
Message-Id: <20180927131904.8826-1-glauber@scylladb.com>

2018-09-27 15:20:31 +02:00

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