Wednesday, June 28, 2006

Making Changes in /proc filesystem

Making changes

Detailing the exact information and usage of each file in /proc is outside the scope of this article. For more information about any /proc files not discussed in this article, one of the best sources is the Linux kernel source itself, which contains some very good documentation. The following files in /proc are more useful to a system administrator. This is not meant to be an exhaustive treatment but an easy-access reference for day-to-day use.


One of the most useful things to learn as a system administrator is how to add more disk space if you have hot-swap drives available to you, without rebooting the system. Without using /proc, you could insert your drive, but you would then have to reboot in order to get the system to recognize the new disk. Here, you can get the system to recognize the new drive with the following command:

echo "scsi add-single-device w x y z" > /proc/scsi/scsi

For this command to work properly, you must get the parameter values w, x, y, and z correct, as follows:

* w is the host adapter ID, where the first adapter is zero (0)
* x is the SCSI channel on the host adaptor, where the first channel is zero (0)
* y is the SCSI ID of the device
* z is the LUN number, where the first LUN is zero (0)

Once your disk has been added to the system, you can mount any previously formatted filesystems or you can start formatting it, and so on. If you are not sure about what device the disk will be, or you want to check any pre-existing partitions, for example, you can use a command such as fdisk -l, which will report this information back to you.

Conversely, the command to remove a device from your system without a reboot would be:

echo "scsi remove-single-device w x y z" > /proc/scsi/scsi

Before you enter this command and remove your hot-swap SCSI disk from your system, make sure you have unmounted any filesystems from this disk first.


This specifies the maximum number of file handles that can be allocated. You may need to increase this value if users get error messages stating that they cannot open more files because the maximum number of open files has been reached. This can be set to any number of files and can be changed by writing a new numeric value to the file.

Default setting: 4096

This file is related to file-max and holds three values:

* Number of allocated file handles
* Number of used file handles
* Maximum number of file handles

This file is read-only and for informational purposes only.

Any files starting with the name "inode" will perform the same operation as files starting with the name "file" as above, but perform their operation relative to inodes instead of file handles.

/proc/sys/fs/overflowuid and /proc/sys/fs/overflowgid
This holds the User ID (UID) and Group ID (GID) for any filesystems that support 16-bit user and group IDs. These values can be changed, but if you really do find the need to do this, you might find it easier to change your group and password file entries instead.

Default Setting: 65534

This specifies the maximum number of super block handlers. Any filesystem you mount needs to use a super block, so you could possibly run out if you mount a lot of filesystems.

Default setting: 256

This shows the currently allocated number of super blocks. This file is read-only and for informational purposes only.


This holds three configurable values that control when process accounting takes place based on the amount of free space (as a percentage) on the filesystem that contains the log:

1. If free space goes below this percentage value then process accounting stops
2. If free space goes above this percentage value then process accounting starts
3. The frequency (in seconds) at which the other two values will be checked

To change a value in this file you should echo a space separated list of numbers.

Default setting: 2 4 30

These values will stop accounting if there is less than 2 percent free space on the filesystem that contains the log and starts it again if there is 4 or more percent free space. Checks are made every 30 seconds.

This file holds a binary value that controls how the system reacts when it receives the ctrl+alt+delete key combination. The two values represent:

1. A zero (0) value means the ctrl+alt+delete is trapped and sent to the init program. This will allow the system to have a graceful shutdown and restart, as if you typed the shutdown command.
2. A one (1) value means the ctrl+alt+delete is not trapped and no clean shutdown will be performed, as if you just turned the power off.

Default setting: 0

This allows you to configure your network domain name. This has no default value and may or may not already be set.

This allows you to configure your network host name. This has no default value and may or may not already be set.

This specifies the maximum size of a message that can be sent from one process to another process. Messages are passed between processes in kernel memory that is not swapped out to disk, so if you increase this value, you will increase the amount of memory used by the operating system.

Default setting: 8192

This specifies the maximum number of bytes in a single message queue.

Default setting: 16384

This specifies the maximum number of message queue identifiers.

Default setting: 16

This represents the amount of time (in seconds) the kernel will wait before rebooting if it reaches a "kernel panic." A setting of zero (0) seconds will disable rebooting on kernel panic.

Default setting: 0

This holds four numeric values that define where logging messages are sent, depending on their importance. For more information on different log levels, read the manpage for syslog(2). The four values of the file are:

1. Console Log Level: messages with a higher priority than this value will be printed to the console
2. Default Message Log Level: messages without a priority will be printed with this priority
3. Minimum Console Log Level: minimum (highest priority) value that the Console Log Level can be set to
4. Default Console Log Level: default value for Console Log Level

Default setting: 6 4 1 7

This is the total amount of shared memory (in bytes) that can be used on the system at any given point.

Default setting: 2097152

This specifies the largest shared memory segment size (in bytes) allowed by the kernel.

Default setting: 33554432

This represents the maximum number of shared memory segments for the whole system.

Default setting: 4096

This activates the System Request Key, if non-zero.

Default setting: 0

This is the maximum number of threads that can be used by the kernel.

Default setting: 2048


This is the time required (in 1/10 seconds) to write a new warning message; other warning messages received during this time will be dropped. This is used to prevent Denial of Service attacks by someone attempting to flood your system with messages.

Default setting: 50 (5 seconds)

This holds a cost value associated with every warning message. The higher the value, the more likely the warning message is to be ignored.

Default setting: 5

This gives the maximum number of packets allowed to queue when an interface receives packets faster than the kernel can process them.

Default setting: 300

This specifies the maximum buffer size allowed per socket.

This is the receive socket buffer's default size (in bytes).

This is the receive socket buffer's maximum size (in bytes).

This is the send socket buffer's default size (in bytes).

This is the send socket buffer's maximum size (in bytes).

All of the IPv4 and IPv6 parameters are fully documented in the kernel source documentation. See the file /usr/src/linux/Documentation/networking/ip-sysctl.txt.

Same as IPv4.


This controls the amount of the total system memory (as a percent) that will be used for buffer memory. It holds three values that can be set by writing a space-separated list to the file:

1. Minimum percentage of memory that should be used for buffers
2. The system will try and maintain this amount of buffer memory when system memory is being pruned in the event of a low amount of system memory remaining
3. Maximum percentage of memory that should be used for buffers

Default setting: 2 10 60

This controls how the system reacts to different levels of free memory. It holds three values that can be set by writing a space-separated list to the file:

1. If the number of free pages in the system reaches this minimum limit, only the kernel will be permitted to allocate any more memory.
2. If the number of free pages in the system falls below this limit, the kernel will start swapping more aggressively to free memory and maintain system performance.
3. The kernel will try to keep this amount of system memory free. Falling below this value will start the kernel swapping.

Default setting: 512 768 1024

This controls how the kernel is allowed to swap memory. It holds three values that can be set by writing a space separated list to the file:

1. Maximum number of pages the kernel tries to free at one time. If you want to increase bandwidth to/from swap, you will need to increase this number.
2. Minimum number of times the kernel tries to free a page on each swap.
3. The number of pages the kernel can write in one swap. This has the greatest impact on system performance. The larger the value, the more data can be swapped and the less time is spent disk seeking. However, a value that is too large will adversely affect system performance by flooding the request queue.

Default setting: 512 32 8

This does the same job as /proc/sys/vm/buffermem, but it does it for memory mapping and generic caching of files.


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