TMPFS(4FS) File Systems TMPFS(4FS)

NAME

tmpfs - memory based file system

SYNOPSIS

#include <sys/mount.h>

mount(const char *special, const char *directory, IMS_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(8) 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 mount_tmpfs(8) for more details.

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(8).

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.

SEE ALSO

mmap(2), mount(2), umount(2), vfstab(5), df(8), mount(8),
mount_tmpfs(8), swap(8)

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(8) 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.

illumos October 9, 1990 illumos