cache — The device cache.

capacity — The capacity of the device, in 512 byte blocks.

driver — The driver and version used to control the device.

geometry — The physical and logical geometry of the device.

media — The type of device, such as a disk.

model — The model name or number of the device.

settings — A collection of current device parameters. This file usually contains quite a bit of useful, technical information. A sample settings file for a standard IDE hard disk looks similar to the following:

name                value          min          max          mode 
----                -----          ---          ---          ---- 
acoustic            0              0            254          rw 
address             0              0            2            rw 
bios_cyl            38752          0            65535        rw 
bios_head           16             0            255          rw 
bios_sect           63             0            63           rw 
bswap               0              0            1            r 
current_speed       68             0            70           rw 
failures            0              0            65535        rw 
init_speed          68             0            70           rw 
io_32bit            0              0            3            rw 
keepsettings        0              0            1            rw 
lun                 0              0            7            rw 
max_failures        1              0            65535        rw 
multcount           16             0            16           rw 
nice1               1              0            1            rw 
nowerr              0              0            1            rw 
number              0              0            3            rw 
pio_mode            write-only     0            255          w 
unmaskirq           0              0            1            rw 
using_dma           1              0            1            rw 
wcache              1              0            1            rw

dentry-state — Provides the status of the directory cache. The file looks similar to the following:

57411	52939	45	0	0	0

The first number reveals the total number of directory cache entries, while the second number displays the number of unused entries. The third number tells the number of seconds between when a directory has been freed and when it can be reclaimed, and the fourth measures the pages currently requested by the system. The last two numbers are not used and display only zeros.

dquot-nr — Lists the maximum number of cached disk quota entries.

file-max — Lists the maximum number of file handles that the kernel allocates. Raising the value in this file can resolve errors caused by a lack of available file handles.

file-nr — Lists the number of allocated file handles, used file handles, and the maximum number of file handles.

overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with file systems that only support 16-bit group and user IDs.

super-max — Controls the maximum number of superblocks available.

super-nr — Displays the current number of superblocks in use.

acct — Controls the suspension of process accounting based on the percentage of free space available on the file system containing the log. By default, the file looks like the following:

4	2	30

The first value dictates the percentage of free space required for logging to resume, while the second value sets the threshold percentage of free space when logging is suspended. The third value sets the interval, in seconds, that the kernel polls the file system to see if logging should be suspended or resumed.

cap-bound — Controls the capability bounding settings, which provides a list of capabilities for any process on the system. If a capability is not listed here, then no process, no matter how privileged, can do it. The idea is to make the system more secure by ensuring that certain things cannot happen, at least beyond a certain point in the boot process.

For a valid list of values for this virtual file, refer to the following installed documentation:

/lib/modules/<kernel-version>/build/include/linux/capability.h.

ctrl-alt-del — Controls whether Ctrl-Alt-Delete gracefully restarts the computer using init (0) or forces an immediate reboot without syncing the dirty buffers to disk (1).

domainname — Configures the system domain name, such as example.com.

exec-shield — Configures the Exec Shield feature of the kernel. Exec Shield provides protection against certain types of buffer overflow attacks.

There are two possible values for this virtual file:

exec-shield-randomize — Enables location randomization of various items in memory. This helps deter potential attackers from locating programs and daemons in memory. Each time a program or daemon starts, it is put into a different memory location each time, never in a static or absolute memory address.

There are two possible values for this virtual file:

hostname — Configures the system hostname, such as www.example.com.

hotplug — Configures the utility to be used when a configuration change is detected by the system. This is primarily used with USB and Cardbus PCI. The default value of /sbin/hotplug should not be changed unless testing a new program to fulfill this role.

modprobe — Sets the location of the program used to load kernel modules. The default value is /sbin/modprobe which means kmod calls it to load the module when a kernel thread calls kmod.

msgmax — Sets the maximum size of any message sent from one process to another and is set to 8192 bytes by default. Be careful when raising this value, as queued messages between processes are stored in non-swappable kernel memory. Any increase in msgmax would increase RAM requirements for the system.

msgmnb — Sets the maximum number of bytes in a single message queue. The default is 16384.

msgmni — Sets the maximum number of message queue identifiers. The default is 16.

osrelease — Lists the Linux kernel release number. This file can only be altered by changing the kernel source and recompiling.

ostype — Displays the type of operating system. By default, this file is set to Linux, and this value can only be changed by changing the kernel source and recompiling.

overflowgid and overflowuid — Defines the fixed group ID and user ID, respectively, for use with system calls on architectures that only support 16-bit group and user IDs.

panic — Defines the number of seconds the kernel postpones rebooting when the system experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting after a panic.

printk — This file controls a variety of settings related to printing or logging error messages. Each error message reported by the kernel has a loglevel associated with it that defines the importance of the message. The loglevel values break down in this order:

Four values are found in the printk file:

6     4     1     7

Each of these values defines a different rule for dealing with error messages. The first value, called the console loglevel, defines the lowest priority of messages printed to the console. (Note that, the lower the priority, the higher the loglevel number.) The second value sets the default loglevel for messages without an explicit loglevel attached to them. The third value sets the lowest possible loglevel configuration for the console loglevel. The last value sets the default value for the console loglevel.

random/ directory — Lists a number of values related to generating random numbers for the kernel.

rtsig-max — Configures the maximum number of POSIX real-time signals that the system may have queued at any one time. The default value is 1024.

rtsig-nr — Lists the current number of POSIX real-time signals queued by the kernel.

sem — Configures semaphore settings within the kernel. A semaphore is a System V IPC object that is used to control utilization of a particular process.

shmall — Sets the total amount of shared memory that can be used at one time on the system, in bytes. By default, this value is 2097152.

shmmax — Sets the largest shared memory segment size allowed by the kernel, in bytes. By default, this value is 33554432. However, the kernel supports much larger values than this.

shmmni — Sets the maximum number of shared memory segments for the whole system, in bytes. By default, this value is 4096

sysrq — Activates the System Request Key, if this value is set to anything other than zero (0), the default.

The System Request Key allows immediate input to the kernel through simple key combinations. For example, the System Request Key can be used to immediately shut down or restart a system, sync all mounted file systems, or dump important information to the console. To initiate a System Request Key, type Alt-SysRq-<system request code> . Replace <system request code> with one of the following system request codes:

This feature is most beneficial when using a development kernel or when experiencing system freezes.

Refer to /usr/share/doc/kernel-doc-<version>/Documentation/sysrq.txt for more information about the System Request Key.

sysrq-key — Defines the key code for the System Request Key (84 is the default).

sysrq-sticky — Defines whether the System Request Key is a chorded key combination. The accepted values are as follows:

sysrq-timer — Specifies the number of seconds allowed to pass before the system request code must be pressed. The default value is 10.

tainted — Indicates whether a non-GPL module is loaded.

threads-max — Sets the maximum number of threads to be used by the kernel, with a default value of 2048.

version — Displays the date and time the kernel was last compiled. The first field in this file, such as #3, relates to the number of times a kernel was built from the source base.

message_burst — Sets the amount of time in tenths of a second required to write a new warning message. This setting is used to mitigate Denial of Service (DoS) attacks. The default setting is 50.

message_cost — Sets a cost on every warning message. The higher the value of this file (default of 5), the more likely the warning message is ignored. This setting is used to mitigate DoS attacks.

The idea of a DoS attack is to bombard the targeted system with requests that generate errors and fill up disk partitions with log files or require all of the system's resources to handle the error logging. The settings in message_burst and message_cost are designed to be modified based on the system's acceptable risk versus the need for comprehensive logging.

netdev_max_backlog — Sets the maximum number of packets allowed to queue when a particular interface receives packets faster than the kernel can process them. The default value for this file is 300.

optmem_max — Configures the maximum ancillary buffer size allowed per socket.

rmem_default — Sets the receive socket buffer default size in bytes.

rmem_max — Sets the receive socket buffer maximum size in bytes.

wmem_default — Sets the send socket buffer default size in bytes.

wmem_max — Sets the send socket buffer maximum size in bytes.

icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate, and icmp_timeexeed_rate — Set the maximum ICMP send packet rate, in 1/100 of a second, to hosts under certain conditions. A setting of 0 removes any delay and is not a good idea.

icmp_echo_ignore_all and icmp_echo_ignore_broadcasts — Allows the kernel to ignore ICMP ECHO packets from every host or only those originating from broadcast and multicast addresses, respectively. A value of 0 allows the kernel to respond, while a value of 1 ignores the packets.

ip_default_ttl — Sets the default Time To Live (TTL), which limits the number of hops a packet may make before reaching its destination. Increasing this value can diminish system performance.

ip_forward — Permits interfaces on the system to forward packets to one other. By default, this file is set to 0. Setting this file to 1 enables network packet forwarding.

ip_local_port_range — Specifies the range of ports to be used by TCP or UDP when a local port is needed. The first number is the lowest port to be used and the second number specifies the highest port. Any systems that expect to require more ports than the default 1024 to 4999 should use a range from 32768 to 61000.

tcp_syn_retries — Provides a limit on the number of times the system re-transmits a SYN packet when attempting to make a connection.

tcp_retries1 — Sets the number of permitted re-transmissions attempting to answer an incoming connection. Default of 3.

tcp_retries2 — Sets the number of permitted re-transmissions of TCP packets. Default of 15.

block_dump — Configures block I/O debugging when enabled. All read/write and block dirtying operations done to files are logged accordingly. This can be useful if diagnosing disk spin up and spin downs for laptop battery conservation. All output when block_dump is enabled can be retrieved via dmesg. The default value is 0.

dirty_background_ratio — Starts background writeback of dirty data at this percentage of total memory, via a pdflush daemon. The default value is 10.

dirty_expire_centisecs — Defines when dirty in-memory data is old enough to be eligible for writeout. Data which has been dirty in-memory for longer than this interval is written out next time a pdflush daemon wakes up. The default value is 3000, expressed in hundredths of a second.

dirty_ratio — Starts active writeback of dirty data at this percentage of total memory for the generator of dirty data, via pdflush. The default value is 40.

dirty_writeback_centisecs — Defines the interval between pdflush daemon wakeups, which periodically writes dirty in-memory data out to disk. The default value is 500, expressed in hundredths of a second.

laptop_mode — Minimizes the number of times that a hard disk needs to spin up by keeping the disk spun down for as long as possible, therefore conserving battery power on laptops. This increases efficiency by combining all future I/O processes together, reducing the frequency of spin ups. The default value is 0, but is automatically enabled in case a battery on a laptop is used.

This value is controlled automatically by the acpid daemon once a user is notified battery power is enabled. No user modifications or interactions are necessary if the laptop supports the ACPI (Advanced Configuration and Power Interface) specification.

For more information, refer to the following installed documentation:

/usr/share/doc/kernel-doc-<version>/Documentation/laptop-mode.txt

lower_zone_protection — Determines how aggressive the kernel is in defending lower memory allocation zones. This is effective when utilized with machines configured with highmem memory space enabled. The default value is 0, no protection at all. All other integer values are in megabytes, and lowmem memory is therefore protected from being allocated by users.

For more information, refer to the following installed documentation:

/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt

max_map_count — Configures the maximum number of memory map areas a process may have. In most cases, the default value of 65536 is appropriate.

min_free_kbytes — Forces the Linux VM (virtual memory manager) to keep a minimum number of kilobytes free. The VM uses this number to compute a pages_min value for each lowmem zone in the system. The default value is in respect to the total memory on the machine.

nr_hugepages — Indicates the current number of configured hugetlb pages in the kernel.

For more information, refer to the following installed documentation:

/usr/share/doc/kernel-doc-<version>/Documentation/vm/hugetlbpage.txt

nr_pdflush_threads — Indicates the number of pdflush daemons that are currently running. This file is read-only, and should not be changed by the user. Under heavy I/O loads, the default value of two is increased by the kernel.

overcommit_memory — Configures the conditions under which a large memory request is accepted or denied. The following three modes are available:

overcommit_ratio — Specifies the percentage of physical RAM considered when /proc/sys/vm/overcommit_memory is set to 2. The default value is 50.

page-cluster — Sets the number of pages read in a single attempt. The default value of 3, which actually relates to 16 pages, is appropriate for most systems.

swappiness — Determines how much a machine should swap. The higher the value, the more swapping occurs. The default value, as a percentage, is set to 60.