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Re: Parallel replication performance
Posted by: Rick James
Date: November 26, 2015 09:57PM

Sorry, I don't know where to find that in dev.mysql.com .

Hardware essentially limits the number of simultaneous I/Os. The code (even from a single CPU) will sometimes queue up multiple I/Os. If there is only one disk drive, only one I/O can happen at a time. Rule of Thumb: 100 operations per second.

With a RAID controller in front of multiple "striped" drives, multiple I/Os can happen, at least for multiple connections.

With InnoDB, writes are (in many cases) delayed. That is, things headed for disk are cached and written latter. Reads are cached, too, but when a connection needs an uncached block, it must wait. While waiting, the CPU is released for other connections.

Think of "parallel replication" as the same as "multiple connections". Hence that _may_ give you some parallelism, hence some speedup. And decrease in "Seconds_behind_master".

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