MySQL Bugs: #46957: Innodb on windows doesn't do concurrent IO per file

Bug #46957	Innodb on windows doesn't do concurrent IO per file
Submitted:	27 Aug 2009 17:15	Modified:	9 Sep 2009 17:53
Reporter:	Mark Callaghan
Status:	Not a Bug
Category:	Server: InnoDB	Severity:	S5 (Performance)
Version:	5.0,5.1,plugin	OS:	Microsoft Windows
Assigned to:		Target Version:
Tags:	windows, innodb, performance

Description:
Innodb uses os_file_read and os_file_write to read and write from files. Both of these
hold a mutex while calling SetFilePointer/WriteFile or SetFilePointer/ReadFile. This is
done because WriteFile and ReadFile apparently do not support positioned reads (pread)
and positioned writes (pwrite) so the mutex is held while the offset to the file is set.

Because of this there can be at most one file IO request pending per file. You really
need to use innodb_file_per_table in this case.

How to repeat:
Read os_file_read and os_file_write. Look for '#ifdef __WIN__'.

Suggested fix:
I don't know.

@Mark .. your synopsis reads "Innodb on windows .." but the OS field reads 'any'. Please
clarify!

set OS to Windows

Mikael Ronstrom might have more background on this. After reading more of the code, I
think Win XP and more recent windows are OK:
>>>
On reading the code again, when os_aio_use_native_aio is set, then async IO is used for
all (most?) IO requests from InnoDB. And it is set on windows as long as
os_get_os_version() != OS_WIN95, OS_WIN31, OS_WINNT. 

So, InnoDB IO performance on Win XP and more recent systems might be better than it is on
Linux.
>>>

Async IO is default on Windows.

Yes, I updated my blog but not the bug after figuring out how that is done. To be
pedantic, this is supported for modern windows (XP or greater) not Windows. The code
which serializes IO is used for older Windows releases.

@Mark .. it was not pedantic.  It was very pedantic!

All Windows versions before XP are *totally dead*. Not supported by Microsoft for several
years. 'Windows' and 'Windows XP and later' are synonyms.