split_doublewrite for each write IO thread

[Vadim Tkachenko]

We discussed with Yasufumi so it may worth to try
split doublewrite area and perform IO into it in multiple IO threads.
currently it is done in single thread but in big size ( I observed 236K IO requests).

While it is good for HDD, it is bad for SSD and we need to experiment with it.

[Yasufumi Kinoshita (yasufumi-kinoshita)]

I imagine "split into dedicated small buffers for each each IO threads" currently.
But each IO requests should be dispatched when requested. I need investigation for detail

[Yasufumi Kinoshita (yasufumi-kinoshita)]

Basically, the doublewrite buffer is needed for SSD?

[Stewart Smith (stewart)]

Leaving unassigned as unlikely to be on Yasufumi's short range TODO (more urgent things).

[Laurynas Biveinis (laurynas-biveinis)]

This is going to be fixed by the parallel doublewrite feature. The fix slightly differs from the original description (work split not at I/O threads but at buffer pool flusher threads)

[Laurynas Biveinis (laurynas-biveinis)]

https://github.com/percona/percona-server/pull/377

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2338