Chapter 2. The

proc

File System

45

controller is communicating with the CD ROM at 20 megabytes per second, while the tape drive is

only connected at 10 megabytes per second.

2.3.9.

/proc/sys/

The

/proc/sys/

directory is different from others in

/proc/

because it not only provides informa 

tion about the system but also allows you to make configuration changes to the kernel. This allows the

administrator of the machine to immediately enable and disable kernel features.

Warning

Use caution when changing settings on a production system using the various files in the /proc/sys/

directory. Changing the wrong setting may render the kernel unstable, requiring a reboot of the sys 

tem.

For this reason, be sure you know the valid options for that file and the expected outcome before

attempting to change a value in /proc/sys/.

A good way to determine if a particular file can configured or is only designed to provide information

is to list it with the  l flag in the terminal. If the file is writable, you may use it to configure the kernel.

For example, a partial listing of

/proc/sys/fs

looks like this:

 r  r  r  

1 root

root

0 May 10 16:14 dentry state

 rw r  r  

1 root

root

0 May 10 16:14 dir notify enable

 r  r  r  

1 root

root

0 May 10 16:14 dquot nr

 rw r  r  

1 root

root

0 May 10 16:14 file max

 r  r  r  

1 root

root

0 May 10 16:14 file nr

In this listing, the files

dir notify enable

and

file max

can be written to and, therefore, can be

used to configure the kernel. The other files only provide feedback on current settings.

Changing a value within a

/proc/sys/

file is done by echoing the new value into the file. For

example, to enable the System Request Key on a running kernel, type the command:

echo 1 > /proc/sys/kernel/sysrq

This will change the

sysrq

file's value from

0

(off) to

1

(on).

The purpose of the System Request Key is to allow you to immediately instruct the kernel to do

a number of important activities by using a simple key combination, such as immediately shutting

down or restarting a system, syncing all mounted file systems, or dumping important information to

your console. This feature is most useful when using a development kernel or if you are experiencing

system freezes. However, it is considered a security risk for an unattended console and is therefore

turned off by default under Red Hat Linux.

Refer to

/usr/src/linux 2.4/Documentation/sysrq.txt

for more information on the System

Request Key.

A few

/proc/sys/

configuration files contain more than one value. In order to correctly send new

values to them, place a space character between each value passed with the

echo

command, such as

is done in this example:

echo 4 2 45 > /proc/sys/kernel/acct