Specifications

Everything in tmpfs is temporary in the sense that no files will be created on any device. If you unmount a
tmpfs instance, everything stored therein is lost.
tmpfs puts everything into the kernel internal caches and grows and shrinks to accommodate the files it
contains and is able to swap unneeded pages out to swap space. It has maximum size limits which can be
adjusted on the fly via 'mount -o remount ...'
If you compare it to ramfs (which was the template to create tmpfs) you gain swapping and limit checking.
Another similar thing is the RAM disk (/dev/ram*), which simulates a fixed size hard disk in physical RAM,
where you have to create an ordinary filesystem on top. Ramdisks cannot swap and you do not have the
possibility to resize them.
9.2.1. Mount Parameters
tmpfs has a couple of mount options:
size: The limit of allocated bytes for this tmpfs instance. The default is half of your physical RAM
without swap. If you oversize your tmpfs instances the machine will deadlock since the OOM handler
will not be able to free that memory.
•
nr_blocks: The same as size, but in blocks of PAGECACHE_SIZE.•
nr_inodes: The maximum number of inodes for this instance. The default is half of the number of
your physical RAM pages.
•
These parameters accept a suffix k, m or g for kilo, mega and giga and can be changed on remount.
To specify the initial root directory you can use the following mount options:
mode: The permissions as an octal number•
uid: The user id•
gid: The group id•
These options do not have any effect on remount. You can change these parameters with chmod(1),
chown(1) and chgrp(1) on a mounted filesystem.
So the following mount command will give you a tmpfs instance on /mytmpfs which can allocate 12MB of
RAM/SWAP and it is only accessible by root.
mount -t tmpfs -o size=12M,mode=700 tmpfs /mytmpfs
9.2.2. Kernel Support for tmpfs
In order to use a tmpfs filesystem, the CONFIG_TMPFS option has to be enabled for your kernel
configuration. It can be found in the Filesystems configuration group. You can simply check if a running
kernel supports tmpfs by searching the contents of /proc/fileysystems:
bash# grep tmpfs /proc/filesystems
nodev tmpfs
bash#
9.2.3. Usage of tmpfs in Embedded Systems
In embedded systems tmpfs is very well suited to provide read and write space (e.g. /tmp and /var) for a
read-only root file system such as CramFs described in section 9.1.4. Compressed ROM Filesystem. One way
to achieve this is to use symbolic links. The following code could be part of the startup file /etc/rc.sh of the
9.2.1. Mount Parameters 107