TMPFS(7FS) File Systems TMPFS(7FS)


NAME


tmpfs - memory based file system

SYNOPSIS


#include <sys/mount.h>

mount (special, directory, MS_DATA, "tmpfs", NULL, 0);


DESCRIPTION


tmpfs is a memory based file system which uses kernel resources relating
to the VM system and page cache as a file system. Once mounted, a tmpfs
file system provides standard file operations and semantics. tmpfs is so
named because files and directories are not preserved across reboot or
unmounts, all files residing on a tmpfs file system that is unmounted
will be lost.


tmpfs file systems can be mounted with the command:

mount -F tmpfs swap directory


Alternatively, to mount a tmpfs file system on /tmp at multi-user
startup time (maximizing possible performance improvements), add the
following line to /etc/vfstab:

swap -/tmp tmpfs - yes -


tmpfs is designed as a performance enhancement which is achieved by
caching the writes to files residing on a tmpfs file system. Performance
improvements are most noticeable when a large number of short lived files
are written and accessed on a tmpfs file system. Large compilations with
tmpfs mounted on /tmp are a good example of this.


Users of tmpfs should be aware of some constraints involved in mounting a
tmpfs file system. The resources used by tmpfs are the same as those used
when commands are executed (for example, swap space allocation). This
means that large sized tmpfs files can affect the amount of space left
over for programs to execute. Likewise, programs requiring large amounts
of memory use up the space available to tmpfs. Users running into this
constraint (for example, running out of space on tmpfs) can allocate more
swap space by using the swap(1M) command.


Another constraint is that the number of files available in a tmpfs file
system is calculated based on the physical memory of the machine and not
the size of the swap device/partition. If you have too many files, tmpfs
will print a warning message and you will be unable to create new files.
You cannot increase this limit by adding swap space.


Normal file system writes are scheduled to be written to a permanent
storage medium along with all control information associated with the
file (for example, modification time, file permissions). tmpfs control
information resides only in memory and never needs to be written to
permanent storage. File data remains in core until memory demands are
sufficient to cause pages associated with tmpfs to be reused at which
time they are copied out to swap.


An additional mount option can be specified to control the size of an
individual tmpfs file system.

SEE ALSO


df(1M), mount(1M), mount_tmpfs(1M), swap(1M), mmap(2), mount(2),
umount(2), vfstab(4)


System Administration Guide: Basic Administration

DIAGNOSTICS


If tmpfs runs out of space, one of the following messages will display in
the console.

directory: File system full, swap space limit exceeded

This message appears because a page could not be allocated while
writing to a file. This can occur if tmpfs is attempting to write
more than it is allowed, or if currently executing programs are using
a lot of memory. To make more space available, remove unnecessary
files, exit from some programs, or allocate more swap space using
swap(1M).


directory: File system full, memory allocation failed

tmpfs ran out of physical memory while attempting to create a new
file or directory. Remove unnecessary files or directories or install
more physical memory.


WARNINGS


Files and directories on a tmpfs file system are not preserved across
reboots or unmounts. Command scripts or programs which count on this will
not work as expected.

NOTES


Compilers do not necessarily use /tmp to write intermediate files
therefore missing some significant performance benefits. This can be
remedied by setting the environment variable TMPDIR to /tmp. Compilers
use the value in this environment variable as the name of the directory
to store intermediate files.


swap to a tmpfs file is not supported.


df(1M) output is of limited accuracy since a tmpfs file system size is
not static and the space available to tmpfs is dependent on the swap
space demands of the entire system.


October 9, 1990 TMPFS(7FS)