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9.2. The TMPFS Virtual Memory Filesystem

The tmpfs filesystem, formerly known as shmfs, is a filesystem keeping all files in virtual memory.

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:

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:

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:

# grep tmpfs /proc/filesystems
nodev   tmpfs

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/ of the read-only ramdisk:

# mkdir /tmpfs
mount -t tmpfs tmpfs /tmpfs
mkdir /tmpfs/tmp /tmpfs/var
# ln -sf /tmpfs/tmp /tmpfs/var /

The commented out sections will of course fail on a read-only root filesystem, so you have to create the /tmpfs mount-point and the symbolic links in your root filesystem beforehand in order to successfully use this setup.

9.1.4. Compressed ROM Filesystem 1. Abstract 9.3. Adding Swap Space
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