File Systems                                           tmpfs(7FS)



NAME
     tmpfs - memory based file system

SYNOPSIS
     #include 

     mount (special, directory, MSDATA, "tmpfs", NUL, 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.




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File Systems                                           tmpfs(7FS)



     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 suffi-
     cient 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),  mounttmpfs(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  allo-
         cated  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




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File Systems                                           tmpfs(7FS)



     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 bene-
     fits. This can be remedied by setting the environment  vari-
     able  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 sys-
     tem  size  is not static and the space available to tmpfs is
     dependent on the swap space demands of the entire system.




































SunOS 5.11           Last change: 9 Oct 1990                    3