Scylla: Remove probabilistic read repair

[duarten]

This is a mirror for CASSANDRA-13910.

While reads are typically sent to just enough replicas to satisfy the CL, read requests may be randomly sent to all replicas in order to repair potentially inconsistent data, based on read_repair_chance and dclocal_read_repair_chance (another exception is speculative retry).

Often this extra load is not justifiable, since inconsistencies are reliably repaired by either QUORUM reads or by hints. When there are dropped hints (something the manager should pay attention to, btw), then issuing a full repair is in order.

Other good arguments are given in the linked issue.

[tzach]

We cant remove it, we invested too much in documenting it!
http://docs.scylladb.com/architecture/anti-entropy/read-repair/

[duarten]

This is a strong indicator that we should document nothing!

[glommer]

On Thu, Jun 7, 2018, 8:56 AM Tzach Livyatan, [e-mail]
wrote:

We cant remove it, we invested too much in documenting it!
http://docs.scylladb.com/architecture/anti-entropy/read-repair/

And ?

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[duarten]

https://en.wikipedia.org/wiki/Joke

[nyh]

Since commit ed0d00f51eda18829aea5bb146e6985a6fb2dc12(a retry of fdd2d9de3d1fb68f96e6a23eb965ad4f957f6805) done for issue #6109, the default of dc_local_read_repair_chance is now 0.
So although probabilistic read repair was not officially "removed" as this issue suggests, or even officially deprecated, it's no longer on unless a user deliberately turns it on.

Also note that the same Cassandra issue, https://issues.apache.org/jira/browse/CASSANDRA-13910, was already resolved more than two years ago, and this options was apparently dropped entirely in Cassandra since 3.11.