mysql 查看函数fsync_sync/fsync/fdatasync的简单比较

此文主要转载自

官网上有关于MySQL的flush method的设置参数说明，但可能很多人不太明白。下文就详细说明此问题。

首先官网的说明如下：

http://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_flush_method

innodb_flush_methodCommand-Line Format–innodb_flush_method=name

Option-File Formatinnodb_flush_method

System Variable Name

Variable ScopeGlobal

Dynamic VariableNo

Permitted Values (<= 5.6.6)

Type (Linux)string

Defaultfdatasync

Valid ValuesO_DSYNC

O_DIRECT

Permitted Values (<= 5.6.6)

Type (HP-UX)string

Defaultfdatasync

Valid ValuesO_DSYNC

O_DIRECT

Permitted Values (<= 5.6.6)

Type (Solaris)string

Defaultfdatasync

Valid ValuesO_DSYNC

O_DIRECT

Permitted Values (>= 5.6.7)

Type (Linux)string

Defaultfdatasync

Valid Valuesfdatasync

O_DSYNC

O_DIRECT

O_DIRECT_NO_FSYNC

Permitted Values (>= 5.6.7)

Type (Solaris)string

Defaultfdatasync

Valid Valuesfdatasync

O_DSYNC

O_DIRECT

O_DIRECT_NO_FSYNC

Permitted Values (>= 5.6.7)

Type (HP-UX)string

Defaultfdatasync

Valid Valuesfdatasync

O_DSYNC

O_DIRECT

O_DIRECT_NO_FSYNC

Controls the system calls used to          flush data to the          InnoDB data

files and log

files, which can influence I/O throughput. This

variable is relevant only for Unix and Linux systems. On

Windows systems, the flush method is always          async_unbuffered and cannot be changed.

By default, InnoDB uses the          fsync() system call to flush both the data

and log files. If          innodb_flush_method option is

set to O_DSYNC, InnoDB

uses O_SYNC to open and flush the log

files, and fsync() to flush the data files.

If O_DIRECT is specified (available on some

GNU/Linux versions, FreeBSD, and Solaris),          InnoDB uses O_DIRECT (or          directio() on Solaris) 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.

An alternative setting is          O_DIRECT_NO_FSYNC: it uses the          O_DIRECT flag during flushing I/O, but

skips the fsync() system call afterwards.

This setting is suitable for some types of filesystems but not

others. For example, it is not suitable for XFS. If you are

not sure whether the filesystem you use requires an          fsync(), for example to preserve all file

metadata, use O_DIRECT instead.

Depending on hardware configuration, setting          innodb_flush_method to          O_DIRECT or          O_DIRECT_NO_FSYNC can have either a

positive or negative effect on performance. Benchmark your

particular configuration to decide which setting to use, or

whether to keep the default. Examine the          Innodb_data_fsyncs status

variable to see the overall number of          fsync() calls done with each setting. The

mix of read and write operations in your workload can also

affect which setting performs better for you. For example, on

a system with a hardware RAID controller and battery-backed

write cache, O_DIRECT can help to avoid

double buffering between the InnoDB buffer

pool and the operating system’s filesystem cache. On some

systems where InnoDB data and log files are

located on a SAN, the default value or          O_DSYNC might be faster for a read-heavy

workload with mostly SELECT statements.

Always test this parameter with the same type of hardware and

workload that reflects your production environment. For

general I/O tuning advice, see          Section 8.5.7, “Optimizing InnoDB Disk I/O”.

Formerly, a value of fdatasync also

specified the default behavior. This value was removed, due to

confusion that a value of fdatasync caused          fsync() system calls rather than          fdatasync() for flushing. To obtain the

default value now, do not set any value for          innodb_flush_method at

startup.

里面提到了fsync()和fdatasync()系统调用，下文给予了详细解释。

之前在研究MySQL的一个参数innodb_flush_method时，就涉及到了fsync/fdatasync这些系统调用[system

call](什么是系统调用？它与库函数的区别在哪？参见这里)。接下来就简单的分析一下sync/fsync/fdatasync的区别。

sync()：int sync( void )这就是它的原型，A call to this function will not

return as long as there is data which has not been written to the

device，sync()同步写，没有写到物理设备就不会返回，但是现实中并不是这样的。在kernel的手册上有解释：BUGS部分(linux中用man查看命令的时候不是都有一个BUGS部分么，就是指的那个)According

to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but may return before the

actual writing is done.  However, since version 1.3.20 Linux does

actually wait.  (This still does not guarantee

data integrity: modern disks have large

caches.)也就是sync()负责将这些写物理设备的请求放入写队列，但是不一定写真正被完成了。

fsync(int fd)：The fsync function can be used to make sure all data

associated with the open file fildes is written to the device associated with

the

descriptor。fsync()负责将一个文件描述符(什么是文件描述符，它是unix、类unix系统打开文件的一种方式，应该相当于打开文件的一个句柄一样)打开的文件写到物理设备，而且是真正的同步写，没有写完成就不会返回，而且写的时候讲文件本身的一些元数据都会更新到物理设备上去，比如atime,mtime等等。

fdatasync(int fd)：When a call to the fdatasync function returns, it

is ensured that all of the file data is written to the

device。它只保证开打文件的数据全部被写到物理设备上，但是一些元数据并不是一定的，这也是它与fsync的区别。

这三个系统调用都简单的介绍完，那么为什么需要它们三个呢？最简单的说是从应用的需求来考虑的，sync是全局的，对整个系统都flush，fsync值针对单个文件，fdatasync当初设计是考虑到有特殊的时候一些基本的元数据比如atime，mtime这些不会对以后读取造成不一致性，因此少了这些元数据的同步可能会在性能上有提升(但fsync和fdatasync两者的性能差别有多大？这个不知道有谁测过没)。所以说三者是根据不同的需求而定的。

接下来谈谈flush dirty

page，也就是前面说的同步写(没写完的话阻塞后面，直到写完才返回)。为什么是刷脏页？脏页表示缓存中的页(一般也就是内存中)也物理设备上的页处于不一致，不一致是由于在内存中被修改。所以为了使内存中的修改持久化到物理磁盘上我们需要将其从内存中flush到物理磁盘上。根据我的理解，一般来说缓存分成这几种：1>应用程序自己带了缓存，比如InnoDB的buffer

pool；2>os层面上的缓存

；3>磁盘设备自己的缓存，比如raid卡一般都管理着自己的缓存；4>磁盘本身或许会有一点点缓存(这个不确定，自己猜想的，这个即使有估计也是极小的)。好了，那么大部分的时候我们说的flush

dirty

page都是指从应用程序的缓存->os的缓存->物理设备，如果物理设备没有缓存的话，此时也就相当于持久化成功，但是像磁盘做了raid，raid卡有缓存的话，实际上还没真正持久化成功，因为此时还只到了raid卡的缓存，没到物理设备，但是由于raid卡一般都带有备用电池，所以即使此时断电也不会造成数据丢失。

刚才说了很多时候应用自己也有缓存机制，那么你是否想过此时与os的缓存有重复呢？答案是：会的。刚才说了我是通过研究MySQL的一个参数innodb_flush_method注意这些的，innodb_flush_method表示flush策略，MySQL提供了fdatasync/O_DSYNC/O_DIRECT这三个选项，默认是fdatasync(详情可参看博文)我这里主要说明为什么会提供选项：O_DIRECT。这个选项告诉os，InnoDB在读写数据的时候都不经过os的缓存，因为刚才说过InnoDB会维护自己的缓存buffer

pool，如果还使用os的缓存那么两者就会有一定的重复。在前面参考的文章里面说O_DIRECT对大量随即读写有效率提升，顺序读写则会下降。所以根据自己的需求来定，不过如果你的MySQL用在是OLTP上，基本上选择O_DIRECT没错。