| TMPFS(4FS) | File Systems | TMPFS(4FS) |
tmpfs — memory
based file system
#include
<sys/mount.h>
mount(const char
*special, const char *directory,
IMS_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 -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(8)
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
mount_tmpfs(8) for more
details.
If tmpfs runs out of space, one of the
following messages will display in the console.
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(8).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.mmap(2), mount(2), umount(2), vfstab(5), df(8), mount(8), mount_tmpfs(8), swap(8)
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(8) 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.
| October 9, 1990 | OmniOS |