mount tmpfs

1 sudo mkdir /mnt/tmpfs

2 sudo mount -t tmpfs -o size=1G none /mnt/tmpfs

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To use a tmpfs, you need to mount it. You can mount your tmpfs directory using the mount command as follows:

```

# mount -t tmpfs -o size=1024m tmpfs /mnt/tmpfs

```

The above command will create a 1024 MB tmpfs directory under the /mnt/tmpfs. You can now use it like any other directory.

To make your system use tmpfs by default, you can edit the /etc/fstab file and add an entry for your tmpfs. 

For example, if your tmpfs is located under the /mnt/tmpfs directory, you can add the following line to fstab:

```tmpfs /mnt/tmpfs tmpfs defaults,noatime,mode=1777,size=1024m 0 0```

Once you save the fstab file, your system will automatically mount the tmpfs directory when it starts up.

The Tmpfs

 

Tmpfs

Tmpfs is a file system which keeps all of its files in virtual memory.

Everything in tmpfs is temporary in the sense that no files will be created on your hard drive. 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.

Since tmpfs lives completely in the page cache and on swap, all tmpfs pages will be shown as “Shmem” in /proc/meminfo and “Shared” in free(1). Notice that these counters also include shared memory (shmem, see ipcs(1)). The most reliable way to get the count is using df(1) and du(1).

tmpfs has the following uses:

1. There is always a kernel internal mount which you will not see at all. This is used for shared anonymous mappings and SYSV shared memory.

   This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not set, the user visible part of tmpfs is not built. But the internal mechanisms are always present.

2. glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for POSIX shared memory (shm_open, shm_unlink). Adding the following line to /etc/fstab should take care of this:

   tmpfs   /dev/shm        tmpfs   defaults        0 0
   

   Remember to create the directory that you intend to mount tmpfs on if necessary.

   This mount is _not_ needed for SYSV shared memory. The internal mount is used for that. (In the 2.3 kernel versions it was necessary to mount the predecessor of tmpfs (shm fs) to use SYSV shared memory.)

3. Some people (including me) find it very convenient to mount it e.g. on /tmp and /var/tmp and have a big swap partition. And now loop mounts of tmpfs files do work, so mkinitrd shipped by most distributions should succeed with a tmpfs /tmp.

4. And probably a lot more I do not know about :-)

tmpfs has three mount options for sizing:

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 PAGE_SIZE.
nr_inodes	The maximum number of inodes for this instance. The default is half of the number of your physical RAM pages, or (on a machine with highmem) the number of lowmem RAM pages, whichever is the lower.

These parameters accept a suffix k, m or g for kilo, mega and giga and can be changed on remount. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your physical RAM: the default, when neither size nor nr_blocks is specified, is size=50%

If nr_blocks=0 (or size=0), blocks will not be limited in that instance; if nr_inodes=0, inodes will not be limited. It is generally unwise to mount with such options, since it allows any user with write access to use up all the memory on the machine; but enhances the scalability of that instance in a system with many CPUs making intensive use of it.

tmpfs has a mount option to set the NUMA memory allocation policy for all files in that instance (if CONFIG_NUMA is enabled) - which can be adjusted on the fly via ‘mount -o remount …’

mpol=default	use the process allocation policy (see set_mempolicy(2))
mpol=prefer:Node	prefers to allocate memory from the given Node
mpol=bind:NodeList	allocates memory only from nodes in NodeList
mpol=interleave	prefers to allocate from each node in turn
mpol=interleave:NodeList	allocates from each node of NodeList in turn
mpol=local	prefers to allocate memory from the local node

NodeList format is a comma-separated list of decimal numbers and ranges, a range being two hyphen-separated decimal numbers, the smallest and largest node numbers in the range. For example, mpol=bind:0-3,5,7,9-15

A memory policy with a valid NodeList will be saved, as specified, for use at file creation time. When a task allocates a file in the file system, the mount option memory policy will be applied with a NodeList, if any, modified by the calling task’s cpuset constraints [See CPUSETS] and any optional flags, listed below. If the resulting NodeLists is the empty set, the effective memory policy for the file will revert to “default” policy.

NUMA memory allocation policies have optional flags that can be used in conjunction with their modes. These optional flags can be specified when tmpfs is mounted by appending them to the mode before the NodeList. See NUMA Memory Policy for a list of all available memory allocation policy mode flags and their effect on memory policy.

=static         is equivalent to        MPOL_F_STATIC_NODES
=relative       is equivalent to        MPOL_F_RELATIVE_NODES

For example, mpol=bind=static:NodeList, is the equivalent of an allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.

Note that trying to mount a tmpfs with an mpol option will fail if the running kernel does not support NUMA; and will fail if its nodelist specifies a node which is not online. If your system relies on that tmpfs being mounted, but from time to time runs a kernel built without NUMA capability (perhaps a safe recovery kernel), or with fewer nodes online, then it is advisable to omit the mpol option from automatic mount options. It can be added later, when the tmpfs is already mounted on MountPoint, by ‘mount -o remount,mpol=Policy:NodeList MountPoint’.

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.

tmpfs has a mount option to select whether it will wrap at 32- or 64-bit inode numbers:

inode64	Use 64-bit inode numbers
inode32	Use 32-bit inode numbers

On a 32-bit kernel, inode32 is implicit, and inode64 is refused at mount time. On a 64-bit kernel, CONFIG_TMPFS_INODE64 sets the default. inode64 avoids the possibility of multiple files with the same inode number on a single device; but risks glibc failing with EOVERFLOW once 33-bit inode numbers are reached - if a long-lived tmpfs is accessed by 32-bit applications so ancient that opening a file larger than 2GiB fails with EINVAL.

So ‘mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs’ will give you tmpfs instance on /mytmpfs which can allocate 10GB RAM/SWAP in 10240 inodes and it is only accessible by root.

Author

Christoph Rohland <cr@sap.com>, 1.12.01

Updated

Hugh Dickins, 4 June 2007

Updated

KOSAKI Motohiro, 16 Mar 2010

Updated

Chris Down, 13 July 2020

 

• ramfs

Ramfs is a very simple FileSystem that exports Linux's disk cacheing mechanisms (the page cache and dentry cache) as a dynamically resizable ram-based filesystem.

Normally all files are cached in memory by Linux. Pages of data read from backing store (usually the ?block_device the filesystem is mounted on) are kept around in case it's needed again, but marked as clean (freeable) in case the Virtual Memory system needs the memory for something else. Similarly, data written to files is marked clean as soon as it has been written to backing store, but kept around for cacheing purposes until the VM reallocates the memory. A similar mechanism (the dentry cache) greatly speeds up access to directories.

With ramfs, there is no backing store. Files written into ramfs allocate dentries and page cache as usual, but there's nowhere to write them to. This means the pages are never marked clean, so they can't be freed by the VM when it's looking to recycle memory.

The amount of code required to implement ramfs is tiny, because all the work is done by the existing Linux cacheing infrastructure. Basically, you're mounting the disk cache as a filesystem. Because of this, ramfs is not an optional component removable via menuconfig, since there would be negligible space savings.

The older "ram disk" mechanism created a synthetic block device out of an area of ram and used it as backing store for a filesystem. This block device was of a fixed size, so the filesystem mounted on it was a fixed size. Using a ram disk also required unnecessarily copying memory from the fake block device into the page cache (and copying changes back out), as well as creating and destroying dentries. Plus it needed a filesystem driver (such as ext2) to format and interpret this data. This wastes memory, creates unnecessary work for the CPU, wastes memory bus bandwidth, and pollutes the CPU caches. (There are tricks to avoid this copying by playing with the page tables, but they're unpleasantly complicated and turn out to be about as expensive as the copying anyway.)

More to the point, all the work ramfs is doing has to happen _anyway_, since all file access goes through the page and dentry caches. The ram disk is simply unnecessary, ramfs is internally much simpler.

One downside of ramfs is you can keep writing data into it until you fill up all memory, and the VM can't free it because the VM thinks that files +should get written to backing store (rather than swap space), but ramfs hasn't got any backing store. Because of this, only root (or a trusted user) should be allowed write access to a ramfs mount.

A ramfs derivative called tmpfs was created to add size limits, and the ability to write the data to swap space. Normal users can be allowed write access to tmpfs mounts. See documentation/filesystems/tmpfs.txt for more information.

See also :

• initramfs

• rootfs

 

Special Filesystems

Linux widely employs the use of special filesystems for certain tasks. These are particularly useful for accessing various kernel data structures and tuning kernel behavior, or for implementing particular functions. 

Note that some of these special filesystems have mount points, such as proc at /proc or sys at /sys and others do not. Examples of special filesystems that have no mount point include sockfs or pipefs; this means user applications don't interact with them, but the kernel uses them, taking advantage of VFS layers and code. These special filesystems are really not true filesystems; they are kernel facilities or subsystems that find the filesystem structural abstraction to be a useful way to recognize data and functionality.

 

Table: Special Filesystems

FILESYSTEM	MOUNT POINT	PURPOSE
rootfs	None	During kernel load, provides an empty root directory
hugetlbfs	Anywhere	Provides extended page access (2 or 4 MB on X86)
bdev	None	Used for block devices
proc	/proc	Pseudofilesystem access to many kernel structures and subsystems
sockfs	None	Used by BSD Sockets
tmpfs	Anywhere	RAM disk with swapping, re-sizing
shm	None	Used by System V IPC Shared Memory
pipefs	None	Used for pipes
binfmt_misc	Anywhere	Used by various executable formats
devpts	/dev/pts	Used by Unix98 pseudo-terminals
usbfs	/proc/bus/usb	Used by USB sub-system for dynamical devices
sysfs	/sys	Used as a device tree
debugfs	/sys/kernel/debug	Used for simple debugging file access