tmpfs man page on SmartOS

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TMPFS(7FS)							    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 relat‐
       ing 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 run‐
       ning 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 unnec‐
	   essary 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.

				  Oct 9, 1990			    TMPFS(7FS)
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