WL#9631: Change InnoDB IO Defaults - innodb_flush_neighbors

Affects: Server-8.0   —   Status: Complete

Since fast IO (SSDs) is now the default for deployment, we should flip the
default of flush_neighbors to be disabled.  This disables merging which may have
a negative impact on performance.

In summary:

SYS-QA will verify that disabling innodb_flush_neighbors by default, doesn't
have performance impact. 

Upgrade Story
This new default changes MySQL to cater for SSDs and fast storage devices by
default.  We expect that for the majority of users, this will result in a small
performance gain.  Users who are using slower hard drives may see a performance
loss, and are encouraged to revert to the previous defaults by setting
FR-1) The compiled default value of innodb_flush_neighbors changed from 1
to 0.

NFR-1) Any changes required to MTR tests.

NFR1) Performance should actually be better on fast IO systems with
flush-neighbors.  It is expected that performance may drop on slow IO systems.