LP #1550404: Percona Server 5.7 manual is misleading for innodb_flush_method

**Reported in Launchpad by Valerii Kravchuk last update 01-03-2016 06:56:41

Manual for Percona Server 5.7 (https://www.percona.com/doc/percona-server/5.7/scalability/innodb_io.html#innodb_flush_method) says:

"ALL_O_DIRECT: use O_DIRECT to open both data and log files, and use fsync() to flush the data files but not the log files. This option is recommended when InnoDB log files are big (more than 8GB), otherwise there might be even a performance degradation. Note: When using this option on ext4 filesystem variable innodb_log_block_size should be set to 4096 (default log-block-size in ext4) in order to avoid the unaligned AIO/DIO warnings."

There are two problems with this text.

First, ALL_O_DIRECT does not work, server does not start with this value set and everything else default (unlike in 5.6), see my last comment to https://bugs.launchpad.net/percona-server/+bug/1131949

Second is that innodb_log_block_size variable it suggests to set does NOT exist in 5.7:

[root@centos ~]# mysql -uroot -e "show variables like 'innodb_log%'"
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[root@centos ~]# mysql -uroot -e "show variables like 'version%'"
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