tmpfs - memory based file system
#include <sys/mount.h> mount (special, directory, MS_DATA, "tmpfs", NULL, 0);
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 –Ftmpfs 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.
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.
If tmpfs runs out of space, one of the following messages will display in the console.
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).
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.
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.
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.