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Subject: [hangout] IBM Article: Use the /proc filesystem to get a handle on your sy
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Use the /proc filesystem to get a handle on your system
Level: Intermediate
http://www-106.ibm.com/developerworks/linux/library/l-adfly.html?ca=den-wud
ftp://www6.software.ibm.com/software/developer/library/l-adfly.pdf
Graham White (gwhite-at-uk.ibm.com)
IT specialist, Hursley, IBM
May 14, 2003
The /proc filesystem is one of Linux's great features, and this article
gives you a thorough grounding in some of its most useful aspects. With it,
you can administer many details of the operating system without ever having
to shut down and reboot the machine, which is a boon for those who need to
keep their systems as available as possible.
Anyone who has administered a system of commercial importance knows the
value of uptime -- or, conversely, knows the headaches you get from users
because of downtime. One of the main reasons a company will run a UNIX
server is because of its reliability and stability. If managed carefully,
there's usually no need to restart these servers for long periods of time.
And to improve matters further, there are administrative tasks -- even at
the kernel level -- that you can perform on the fly, keeping your servers
available. While you may still need to restart a system to upgrade hardware
or if someone trips over the power cord, it's good to know that many
administrative tasks can be performed without disrupting service.
This article includes hints and tips for performing various administrative
tasks and changing your system without rebooting. Linux provides various
ways to change underlying operating system values and settings while keeping
the system up and running. These come in two basic forms, those that are
general to all Linux systems and are provided in the Linux kernel (you can
find more information about the Linux kernel and download kernel source at
the Linux Kernel Archives; see Resources for a link), and those that are
distribution specific and provided by the vendor. This article deals with
both types.
Note: This article is written for a 2.4-level kernel. Some of the options
and features may be different for other kernel versions.
Changing running kernel parameters
Linux provides a really neat way for administrators to change the kernel
while the system is running and without the need to reboot the
kernel/system. This is done with a virtual filesystem called /proc. Linux
Gazette provides one of the simplest and easiest references on /proc I have
seen. (See Resources for a link.) Very basically, the /proc filesystem gives
you a view into the running kernel, which can be useful for monitoring
performance, discovering system information, finding out how the system is
configured, and changing that configuration. This filesystem is called a
virtual filesystem, because it is not really a filesystem at all. It's just
a map provided by the kernel that is attached to your usual filesystem
structure to give you access to it.
The fact that we have some way of changing the running kernel parameters
while the system is up and running gives the system administrator great
power and flexibility in changing kernel settings. This sort of
implementation was an inspired idea on the part of the Linux kernel
developers. But can too much power be a bad thing? Sometimes. If you are
going to change anything in the /proc filesystem, you must make sure that
you know what you are changing and what effect this will have on the system.
These are really useful techniques, but a wrong move can give you some
rather undesired consequences. If you are new to this sort of thing or are
not sure what effect one of your changes will have, practice on a machine
that is not important to you or your business.
How to make changes
First, think about how not to make changes to the kernel. There are two good
reasons why you should not just jump into the /proc filesystem, open a file
in your text editor, make a bunch of changes, and save the file back out
again. These are:
Data integrity: All of these files represent the running system, and since
the kernel may change any of these files at any time, if you open an editor
and change some data while the system is changing it underneath you,
whatever you save back is unlikely to be what the kernel is expecting.
Virtual files: All of these files do not actually exist. How would the saved
data be synchronized, etc.?
The answer to making changes to any of these files, therefore, is not to use
an editor. When making changes to anything at all in the /proc filesystem,
you should use the echo command and redirect the output from the command
line into your chosen files under /proc. For example:
echo "Your-New-Kernel-Value" > /proc/your/file
Similarly, if you wish to view information from /proc, you should either use
a command that is designed for the purpose or use the command line cat
command.
What to change
You do not need to be a kernel hacker to get good use out of /proc, and a
basic understanding of the structure of this filesystem will aid you
greatly. You may find that you don't need to know about anything in /proc,
until the day a user asks you for a certain bit of functionality that makes
you glad you bothered to learn where to look to make changes. The /proc
filesystem helps the system administrator in this respect via its structure
and file permissions.
Each file in /proc has a very particular set of file permissions assigned to
it and will be owned by a particular user ID. This is very carefully done so
that the correct functionality is presented to the administrator and to the
users. The following list summarizes what particular permissions may do on
individual files:
Read-only: File is not changeable by any user; used for presenting system
information
Root-write: If a file is writeable in /proc, it is usually writeable only by
the root user
Root-read: Some files may not be viewable to normal system users, only to
root
Other: You may find combinations other than the common three, above, for
various reasons
A very broad generalization about /proc is that you will find most of it
read-only except for the /proc/sys directory. This directory is the one that
holds most kernel parameters (rather than information) and is the one that
is designed to be changed while the system is running. As a result, this is
the directory that this article will look mainly at.
The last thing to know about learning what to change in /proc is what you
should actually be writing to these files. You will notice as you look at
various files in /proc that some of them are human readable and some are
data files. The data files can still be read by using specific utilities
such as top, lspci, and free. You will also notice that the human-readable
files take two different formats: some are binary switches and others
contain more information. The binary switch files only contain a zero (off)
or a one (on) for that particular kernel feature.
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.
/proc/scsi
/proc/scsi/scsi
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.
/proc/sys/fs/
/proc/sys/fs/file-max
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
/proc/sys/fs/file-nr
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.
/proc/sys/fs/inode-*
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
/proc/sys/fs/super-max
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
/proc/sys/fs/super-nr
This shows the currently allocated number of super blocks. This file is
read-only and for informational purposes only.
/proc/sys/kernel
/proc/sys/kernel/acct
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:
If free space goes below this percentage value then process accounting stops
If free space goes above this percentage value then process accounting
starts
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.
/proc/sys/kernel/ctrl-alt-del
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:
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.
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
/proc/sys/kernel/domainname
This allows you to configure your network domain name. This has no default
value and may or may not already be set.
/proc/sys/kernel/hostname
This allows you to configure your network host name. This has no default
value and may or may not already be set.
/proc/sys/kernel/msgmax
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
/proc/sys/kernel/msgmnb
This specifies the maximum number of bytes in a single message queue.
Default setting: 16384
/proc/sys/kernel/msgmni
This specifies the maximum number of message queue identifiers.
Default setting: 16
/proc/sys/kernel/panic
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
/proc/sys/kernel/printk
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:
Console Log Level: messages with a higher priority than this value will be
printed to the console
Default Message Log Level: messages without a priority will be printed with
this priority
Minimum Console Log Level: minimum (highest priority) value that the Console
Log Level can be set to
Default Console Log Level: default value for Console Log Level
Default setting: 6 4 1 7
/proc/sys/kernel/shmall
This is the total amount of shared memory (in bytes) that can be used on the
system at any given point.
Default setting: 2097152
/proc/sys/kernel/shmax
This specifies the largest shared memory segment size (in bytes) allowed by
the kernel.
Default setting: 33554432
/proc/sys/kernel/shmmni
This represents the maximum number of shared memory segments for the whole
system.
Default setting: 4096
/proc/sys/kernel/sysrq
This activates the System Request Key, if non-zero.
Default setting: 0
/proc/sys/kernel/threads-max
This is the maximum number of threads that can be used by the kernel.
Default setting: 2048
/proc/sys/net
/proc/sys/net/core/message_burst
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)
/proc/sys/net/core/message_cost
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
/proc/sys/net/core/netdev_max_backlog
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
/proc/sys/net/core/optmem_max
This specifies the maximum buffer size allowed per socket.
/proc/sys/net/core/rmem_default
This is the receive socket buffer's default size (in bytes).
/proc/sys/net/core/rmem_max
This is the receive socket buffer's maximum size (in bytes).
/proc/sys/net/core/wmem_default
This is the send socket buffer's default size (in bytes).
/proc/sys/net/core/wmem_max
This is the send socket buffer's maximum size (in bytes).
/proc/sys/net/ipv4
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.
/proc/sys/net/ipv6
Same as IPv4.
/proc/sys/vm
/proc/sys/vm/buffermem
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:
Minimum percentage of memory that should be used for buffers
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
Maximum percentage of memory that should be used for buffers
Default setting: 2 10 60
/proc/sys/vm/freepages
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:
If the number of free pages in the system reaches this minimum limit, only
the kernel will be permitted to allocate any more memory.
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.
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
/proc/sys/vm/kswapd
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:
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.
Minimum number of times the kernel tries to free a page on each swap.
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
/proc/sys/vm/pagecache
This does the same job as /proc/sys/vm/buffermem, but it does it for memory
mapping and generic caching of files.
Making your kernel settings persistent
A handy utility is provided for making changes to any kernel parameters
under the /proc/sys directory. It allows you to make changes to the running
kernel (similarly to the echo and redirection method used above), but it
also has a configuration file that is executed on system boot. This lets you
make changes to the running kernel and add them to the configuration file so
that any changes you make will remain after a system reboot.
The utility is called sysctl and is fully documented in the man pages at
sysctl(8). The configuration file for sysctl is /etc/sysctl.conf, which can
be edited and is documented under sysctl.conf(8). Sysctl treats the files
under /proc/sys as individual variables that can be changed. So, for
example, the file under /proc/sys that represents the maximum number of file
handles allowed on the system, /proc/sys/fs/file-max, is represented as
fs.file-max.
This example reveals some oddities in sysctl notation. Since sysctl can only
change variables under the /proc/sys directory, that part of the variable
name is missing as the variables are always assumed to be under that
directory. The next change to note is that the directory separators (slash,
/) have changed to periods (dot, .).
There are two simple rules for converting between files in /proc/sys and
variables in sysctl:
Drop the /proc/sys from the beginning.
Swap slashes for dots in the filenames.
These two rules will let you swap any file name in /proc/sys for any
variable name in sysctl. The general file to variable conversion is:
/proc/sys/dir/file --> dir.file
dir1.dir2.file --> /proc/sys/dir1/dir2/file
You can view all the variables that are available to be changed, along with
their current setting, using the command sysctl -a.
Variables can also be changed using sysctl, which does exactly the same job
as the echo method used above. This notation is as follows:
sysctl -w dir.file="value"
Using the file-max example again, we could change this value to 16384 using
one of two methods as follows:
sysctl -w fs.file-max="16384"
Or:
echo "16384" > /proc/sys/fs/file-max
Don't forget that sysctl does not add changes made to the configuration
file; this is left for you to do manually. If you want your changes to
persist after a reboot, you must maintain this file.
Note: Not all distributions provide sysctl support. If this is the case for
your particular system, then you can use the echo and redirect method
described above and add these commands to a start-up script so they are
executed every time the system boots.
Commands for setting the system
It is possible to change other non-kernel system parameters while the system
is running and also get these settings to take effect without rebooting.
These can mainly be classified as services, daemons, and servers that will
be listed in the /etc/init.d directory. Since there is an increasingly wide
range of scripts that can be listed in this directory, it is not possible to
go through all the different configurations here. However, below are a few
examples of how the /etc/init.d scripts can be manipulated on different
distributions of Linux. Examples of where changes to a daemon and a reload
of the configuration without rebooting might be useful are:
Changing your Web server configuration and reloading Apache
Removing an inetd login service that you don't require
Manipulating your network settings
Exporting new filesystems via NFS
Starting/stopping your firewall
First, the generic way of manipulating system services is directly, via the
scripts in /etc/init.d. These scripts take parameters to manipulate the
services that they control; you can type the script name without any
parameters to see what the valid options are. Common parameters are:
start: Starts a stopped service
stop: Stops a running service
restart: Stops and then starts a running service; will start a stopped
service
reload: Reloads service configuration without breaking any connection(s)
status: Outputs whether the service is running or not
As an example, the following command would reload your xinetd configuration
without terminating any connected user's sessions (useful if you make a
change to /etc/xinetd.conf):
/etc/init.d/xinetd reload
Red Hat provides a command, service, that will manipulate services for you.
The service command provides the same functionality as typing the script
name itself. The syntax is as follows:
service script-name [parameter]
For example:
service xinetd reload
SuSE also provide a command called rc. This is similar to the service
command above, but has no space between the command and the script name. The
syntax is as follows:
rc{script-name} parameter
For example:
rcapache start
Similarly to changing kernel parameters, once you reboot your system, any
changes made to services will be lost. More and more distributions are
adopting the use of the chkconfig command, which manages the services that
are started at various run levels (including on boot). At the time of this
writing, the chkconfig command syntax differs slightly on different versions
of Linux, but if you enter the command chkconfig without any parameters, you
will get a list of how to use it. More information about chkconfig can also
be found via the man pages at chkconfig(8).
Conclusion
Configuring the Linux kernel on the fly using the /proc filesystem isn't to
be taken lightly, but once you understand its structure and how to
manipulate the various files and parameters, you've gained the use of a
powerful tool for keeping your servers available around the clock.
Acknowledgment
I would like to thank Mr. Adrian Fewings for proofreading this article.
Resources
Download, get involved with, or just learn about the Linux kernel at the
Linux Kernel Archives.
Refer to the kernel documentation in the "Documentation" directory where you
installed the kernel source.
Learn more about the /proc virtual filesystem from Linux Gazette.
Read the man pages for sysctl(8) and sysctl.conf(8).
Find more Linux documents at the Linux Documentation Project homepage.
For a guide to troubleshooting hardware problems, read "Linux hardware
stability guide, Part 1" (developerWorks, March 2001) and "Linux hardware
stability guide, Part 2" (developerWorks, July 2001).
"Understanding Linux configuration files" (developerWorks, December 2001)
gives an overview of files that control permissions, system applications,
daemons, and more.
"Configuring and compiling the kernel" (developerWorks, May 2002) takes you
step by step through the basic task of compiling the Linux kernel from
sources.
If you want to know what "high availability" means from a mainframe
perspective, read the IBM Redpaper "Linux on IBM zSeries and S/390: High
Availability for z/VM and Linux."
Find more resources for Linux developers in the developerWorks Linux zone.
About the author
Graham graduated from The University of Exeter with a B.Sc. (Honors) degree
in Computer Science with Management Science in July 2000. He joined IBM as
an IT support worker in September 2000 with no previous experience and
started learning Linux. One year later, in September 2001, he was certified
as a Red Hat Certified Engineer. His work and personal interests gave him
experience with many different versions of Linux running on a wide range of
platforms to support the development community at the IBM Hursley Laboratory
in the UK. Recently, he has taken up writing articles about Linux, his first
and only other publication being a guide for the Linux Documentation
Project. Contact Graham at gwhite at uk.ibm.com.
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