Linux核心的檔案預讀readahead，

Linux核心的檔案預讀readahead，

Linux的檔案預讀readahead，指Linux系統核心將指定檔案的某地區預讀進頁緩衝起來，便於接下來對該地區進行讀取時，不會因缺頁（page fault）而阻塞。因為從記憶體讀取比從磁碟讀取要快很多。預讀可以有效減少磁碟的尋道次數和應用程式的I/O等待時間，是改進磁碟讀I/O效能的重要最佳化手段之一。

維基百科上關於readhead的介紹資料：

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、設定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

查看磁碟的預讀扇區

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

256

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

256

[root@localhost ~]# 

 

設定磁碟的預讀扇區

[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 ~]# 

 

必須將其寫入設定檔/etc/rc.local，否則重啟就會失效。

[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

 

參考資料：

https://en.wikipedia.org/wiki/Readahead

http://www.hustyx.com/cpp/96/