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.Dd October 9, 1990
.Dt TMPFS 4FS
.Os
.Sh NAME
.Nm tmpfs
.Nd memory based file system
.Sh SYNOPSIS
.In sys/mount.h
.Fo mount
.Fa "const char *special"
.Fa "const char *directory"
.Fa IMS_DATA
.Fa \(dqtmpfs\(dq
.Fa NULL
.Fa 0
.Fc
.Sh DESCRIPTION
.Nm
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
.Nm
file system provides standard file operations and semantics.
.Nm
is so named because files and directories are not preserved across
reboot or unmounts, all files residing on a
.Nm
file system that is unmounted will be lost.
.Pp
.Nm
file systems can be mounted with the command:
.Bd -literal -offset indent
mount -F tmpfs swap directory
.Ed
.Pp
Alternatively, to mount a
.Nm
file system on
.Pa /tmp
at multi-user startup time
.Pq maximizing possible performance improvements  ,
add the following line to
.Pa /etc/vfstab :
.Bd -literal -offset indent
swap \(mi/tmp tmpfs \(mi yes \(mi
.Ed
.Pp
.Nm
is designed as a performance enhancement which is achieved by caching the writes
to files residing on a
.Nm
file system.
Performance improvements are most noticeable when a large number of short lived
files are written and accessed on a
.Nm
file system.
Large compilations with
.Nm
mounted on
.Pa /tmp
are a good example of this.
.Pp
Users of
.Nm
should be aware of some constraints involved in mounting a
.Nm
file system.
The resources used by
.Nm
are the same as those used when commands are executed
.Pq for example, swap space allocation .
This means that large sized
.Nm
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
.Nm
Users running into this constraint
.Po
for example, running out of space on
.Nm
.Pc
can allocate more swap space by using the
.Xr swap 8
command.
.Pp
Another constraint is that the number of files available in a
.Nm
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,
.Nm
will print a warning message and you will be unable to create new files.
You cannot increase this limit by adding swap space.
.Pp
Normal file system writes are scheduled to be written to a permanent storage
medium along with all control information associated with the file
.Pq for example, modification time, file permissions .
.Nm
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
.Nm
to be reused at which time they are copied out to swap.
.Pp
An additional mount option can be specified to control the size of an individual
.Nm
file system.
See
.Xr mount_tmpfs 8
for more details.
.Sh DIAGNOSTICS
If
.Nm
runs out of space, one of the following messages will display in the console.
.Bl -tag -width Ds
.It "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
.Nm
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
.Xr swap 8 .
.It "directory: File system full, memory allocation failed"
.Nm
ran out of physical memory while attempting to create a new file or
directory.
Remove unnecessary files or directories or install more physical memory.
.El
.Sh SEE ALSO
.Xr mmap 2 ,
.Xr mount 2 ,
.Xr umount 2 ,
.Xr vfstab 5 ,
.Xr df 8 ,
.Xr mount 8 ,
.Xr mount_tmpfs 8 ,
.Xr swap 8
.Sh WARNINGS
Files and directories on a
.Nm
file system are not preserved across reboots or unmounts.
Command scripts or programs which count on this will not work as expected.
.Sh 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
.Ev TMPDIR to
.Pa /tmp .
Compilers use
the value in this environment variable as the name of the directory to store
intermediate files.
.Pp
.Sy swap
to a
.Nm
file is not supported.
.Pp
.Xr df 8
output is of limited accuracy since a
.Nm
file system size
is not static and the space available to
.Nm
is dependent on the swap space demands of the entire system.