Technical data

tmpfs(7FS) File Systems SunOS 5.5
NAME tmpfs − memory based filesystem
SYNOPSIS #include <sys/mount.h>
mount (special, directory, MS_DATA, "tmpfs", NULL, 0);
DESCRIPTION tmpfs is a memory based filesystem which uses kernel resources relating to theVM sys-
tem and page cache as a filesystem. Once mounted, a tmpfs filesystem provides stan-
dard 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 filesystem that is
unmounted will be lost.
tmpfs filesystems can be mounted with the command:
mount −F tmpfs swap directory
Alternatively, to mount a tmpfs filesystem on /tmp at multi-user startup time (and max-
imizing 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 filesystem. Performance improvements are most noticeable
when a large number of short lived files are written and accessed on a tmpfs filesystem.
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
filesystem. 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, runningout 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 filesystem 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 warningmessage and
you will be unableto create new files. You cannot increase this limit by adding swap
space.
Normal filesystem 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
filesystem.
7FS-376 modified 9 Oct 1990