Per Session innodb_flush_log_at_trx_commit poorly documented

[Jaime Crespo]

This and the same section on 5.5 is poorly written and has some documentation bugs:
http://www.percona.com/doc/percona-server/5.6/scalability/innodb_io.html#innodb_use_global_flush_log_at_trx_commit

In summary:

* The feature doc is difficult to understand, and overuses "GLOBAL" when it means "session as if it was set globally"
* I think it has some erroneous information: wrong quoted example and the session variable can always be set, but (hopefully) it will have no effect
* The feature is not documented in any other place, and it requires further discussion than simple parameters settings

I have written an alternative version (please check validity) to at least change the current information:
---------------

This variable is used to enable or disable the effect of the per session value of innodb_flush_log_at_trx_commit.

If the global variable innodb_use_global_flush_log_at_trx_commit is set to 1 (True), the session will use always use the current global value of innodb_flush_log_at_trx_commit. This is the default value.

Under this mode, changing the session variable for innodb_flush_log_at_trx_commit will have no effect for upstream compatibility.

If the global variable innodb_use_global_flush_log_at_trx_commit is set to 0 (False), the user can modify the MySQL commit durability per session, using:

SET SESSION innodb_flush_log_at_trx_commit=N

This change will only affect the transactions in the session where it has changed, while the rest will keep the global innodb_flush_log_at_trx_commit value unless similarly changed.

[Laurynas Biveinis (laurynas-biveinis)]

I agree that innodb_use_global_flush_log_at_trx_commit docs should be improved. The 5.1 innodb_flush_log_at_trx_commit_session docs are IMHO clearer but still targeting the bug to 5.1 too for a review (e.g. is innodb_flush_log_at_trx_commit_session really a global and not session variable?)

[Jaime Crespo (jynus)]

For future reference, enabling a feature by setting it from 1 to 0 is anti-intuitive. I understand the original requirement of maintaining backwards compatibility, but setting innodb_use_session_flush_log_at_trx_commit to 1 would have been more intuitive.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1466