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.  Perfor-
       mance improvements are most noticeable when a large number of short lived files are written and accessed on a tmpfs file system. Large com-
       pilations 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 direc-
       tory 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.

SunOS 5.11							    9 Oct 1990								tmpfs(7FS)