Linux内核的文件预读readahead

Linux的文件预读readahead，指Linux系统内核将指定文件的某区域预读进页缓存起来，便于接下来对该区域进行读取时，不会因缺页（page fault）而阻塞。因为从内存读取比从磁盘读取要快很多。预读可以有效的减少磁盘的寻道次数和应用程序的I/O等待时间，是改进磁盘读I/O性能的重要优化手段之一。

维基百科上关于readhead的介绍资料：

readahead is a system call of the Linux kernel that loads a file's contents into the page cache, PRoviding that way a file prefetching technology. When a file is subsequently accessed, its contents are read from the main memory (RAM) rather than from a hard disk drive (HDD), resulting in much lower file access latencies due to much higher performance of the main memory.[1][2]

Many Linux distributions use readahead on a list of commonly used files to speed up booting. In such a setup, if the kernel is booted with the profile boot parameter, it will record all file accesses during bootup and write a new list of files to be read during later boot sequences. This will make additional installed services start faster, because they are not included in the default readahead list.[3]

In Linux distributions that use systemd, readahead binary (as part of the boot sequence) is replaced by systemd-readahead.[4][5] However, support for readahead was removed from systemd in its version 219, being described as unmaintained and unable to provide expected performance benefits.[6]

Certain experimental page-level prefetching systems have been developed to further improve performance.[7]

查看、设置readhead使用命令blockdev。如下所示：

[root@localhost ~]# /sbin/blockdev

Usage:

 blockdev -V

 blockdev --report [devices]

 blockdev [-v|-q] commands devices

Available commands:

 --getsz get size in 512-byte sectors

 --setro set read-only

 --setrw set read-write

 --getro get read-only

 --getss get logical block (sector) size

 --getpbsz get physical block (sector) size

 --getiomin get minimum I/O size

 --getioopt get optimal I/O size

 --getalignoff get alignment offset

 --getmaxsect get max sectors per request

 --getbsz get blocksize

 --setbsz BLOCKSIZE set blocksize on file descriptor opening the block device

 --getsize get 32-bit sector count

 --getsize64 get size in bytes

 --setra READAHEAD set readahead

 --getra get readahead

 --setfra FSREADAHEAD set filesystem readahead

 --getfra get filesystem readahead

 --flushbufs flush buffers

 --rereadpt reread partition table

查看磁盘的预读扇区

[root@localhost ~]# /sbin/blockdev --getra /dev/sda

256

[root@localhost ~]# /sbin/blockdev --getra /dev/sdb

256

[root@localhost ~]# 

设置磁盘的预读扇区

[root@localhost ~]# /sbin/blockdev --getra /dev/sda

256

[root@localhost ~]# /sbin/blockdev --setra 2048 /dev/sda

[root@localhost ~]# /sbin/blockdev --getra /dev/sda

2048

[root@localhost ~]# /sbin/blockdev --getra /dev/sdb

256

[root@localhost ~]# /sbin/blockdev --setra 2048 /dev/sdb

[root@localhost ~]# /sbin/blockdev --getra /dev/sdb

2048

[root@localhost ~]# 

必须将其写入配置文件/etc/rc.local，否则重启就会失效。

[root@localhost ~]# echo '/sbin/blockdev --setra 2048 /dev/sda' >> /etc/rc.local

[root@localhost ~]# more /etc/rc.local

#!/bin/sh

#

# This script will be executed *after* all the other init scripts.

# You can put your own initialization stuff in here if you don't

# want to do the full Sys V style init stuff.

touch /var/lock/subsys/local

/sbin/blockdev --setra 2048 /dev/sda

[root@localhost ~]# echo '/sbin/blockdev --setra 2048 /dev/sdb' >> /etc/rc.local

[root@localhost ~]# more /etc/rc.local

#!/bin/sh

#

# This script will be executed *after* all the other init scripts.

# You can put your own initialization stuff in here if you don't

# want to do the full Sys V style init stuff.

touch /var/lock/subsys/local

/sbin/blockdev --setra 2048 /dev/sda

/sbin/blockdev --setra 2048 /dev/sdb