Description:
The innodb_flush_method documentation doesn't describe all possible settings for the option.
innodb_flush_method
If set to fdatasync (the default), InnoDB uses fsync() to flush both the data and log files. If set to O_DSYNC, InnoDB uses O_SYNC to open and flush the log files, but uses fsync() to flush the data files. If O_DIRECT is specified (available on some GNU/Linux versions), InnoDB uses O_DIRECT to open the data files, and uses fsync() to flush both the data and log files. Note that InnoDB uses fsync() instead of fdatasync(), and it does not use O_DSYNC by default because there have been problems with it on many varieties of Unix. This variable is relevant only for Unix. On Windows, the flush method is always async_unbuffered and cannot be changed.
Different values of this variable can have a marked effect on InnoDB performance. For example, on some systems where InnoDB data and log files are located on a SAN, it has been found that setting innodb_flush_method to O_DIRECT can degrade performance of simple SELECT statements by a factor of three.
How to repeat:
Look in srv0start.c
#ifndef __WIN__
} else if (0 == ut_strcmp(srv_file_flush_method_str, "fdatasync")) {
srv_unix_file_flush_method = SRV_UNIX_FDATASYNC;
} else if (0 == ut_strcmp(srv_file_flush_method_str, "O_DSYNC")) {
srv_unix_file_flush_method = SRV_UNIX_O_DSYNC;
} else if (0 == ut_strcmp(srv_file_flush_method_str, "O_DIRECT")) {
srv_unix_file_flush_method = SRV_UNIX_O_DIRECT;
} else if (0 == ut_strcmp(srv_file_flush_method_str, "littlesync")) {
srv_unix_file_flush_method = SRV_UNIX_LITTLESYNC;
} else if (0 == ut_strcmp(srv_file_flush_method_str, "nosync")) {
srv_unix_file_flush_method = SRV_UNIX_NOSYNC;
#else
} else if (0 == ut_strcmp(srv_file_flush_method_str, "normal")) {
srv_win_file_flush_method = SRV_WIN_IO_NORMAL;
os_aio_use_native_aio = FALSE;
} else if (0 == ut_strcmp(srv_file_flush_method_str, "unbuffered")) {
srv_win_file_flush_method = SRV_WIN_IO_UNBUFFERED;
os_aio_use_native_aio = FALSE;
} else if (0 == ut_strcmp(srv_file_flush_method_str,
"async_unbuffered")) {
srv_win_file_flush_method = SRV_WIN_IO_UNBUFFERED;
#endif
} else {
fprintf(stderr,
"InnoDB: Unrecognized value %s for innodb_flush_method\n",
srv_file_flush_method_str);
return(DB_ERROR);
}
Suggested fix:
It's not really a good idea to have undocumented settings, so either document them, and add a warning they shouldn't be used, or removed them completely?
Description: The innodb_flush_method documentation doesn't describe all possible settings for the option. innodb_flush_method If set to fdatasync (the default), InnoDB uses fsync() to flush both the data and log files. If set to O_DSYNC, InnoDB uses O_SYNC to open and flush the log files, but uses fsync() to flush the data files. If O_DIRECT is specified (available on some GNU/Linux versions), InnoDB uses O_DIRECT to open the data files, and uses fsync() to flush both the data and log files. Note that InnoDB uses fsync() instead of fdatasync(), and it does not use O_DSYNC by default because there have been problems with it on many varieties of Unix. This variable is relevant only for Unix. On Windows, the flush method is always async_unbuffered and cannot be changed. Different values of this variable can have a marked effect on InnoDB performance. For example, on some systems where InnoDB data and log files are located on a SAN, it has been found that setting innodb_flush_method to O_DIRECT can degrade performance of simple SELECT statements by a factor of three. How to repeat: Look in srv0start.c #ifndef __WIN__ } else if (0 == ut_strcmp(srv_file_flush_method_str, "fdatasync")) { srv_unix_file_flush_method = SRV_UNIX_FDATASYNC; } else if (0 == ut_strcmp(srv_file_flush_method_str, "O_DSYNC")) { srv_unix_file_flush_method = SRV_UNIX_O_DSYNC; } else if (0 == ut_strcmp(srv_file_flush_method_str, "O_DIRECT")) { srv_unix_file_flush_method = SRV_UNIX_O_DIRECT; } else if (0 == ut_strcmp(srv_file_flush_method_str, "littlesync")) { srv_unix_file_flush_method = SRV_UNIX_LITTLESYNC; } else if (0 == ut_strcmp(srv_file_flush_method_str, "nosync")) { srv_unix_file_flush_method = SRV_UNIX_NOSYNC; #else } else if (0 == ut_strcmp(srv_file_flush_method_str, "normal")) { srv_win_file_flush_method = SRV_WIN_IO_NORMAL; os_aio_use_native_aio = FALSE; } else if (0 == ut_strcmp(srv_file_flush_method_str, "unbuffered")) { srv_win_file_flush_method = SRV_WIN_IO_UNBUFFERED; os_aio_use_native_aio = FALSE; } else if (0 == ut_strcmp(srv_file_flush_method_str, "async_unbuffered")) { srv_win_file_flush_method = SRV_WIN_IO_UNBUFFERED; #endif } else { fprintf(stderr, "InnoDB: Unrecognized value %s for innodb_flush_method\n", srv_file_flush_method_str); return(DB_ERROR); } Suggested fix: It's not really a good idea to have undocumented settings, so either document them, and add a warning they shouldn't be used, or removed them completely?