137 lines
5.9 KiB
Plaintext
137 lines
5.9 KiB
Plaintext
Tmpfs is a file system which keeps all 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 currently in memory will show up as cached. It will not show up
|
|
as shared or something like that. Further on you can check the actual
|
|
RAM+swap use of a tmpfs instance with 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 build. 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_CACHE_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 prefers to allocate memory from the local node
|
|
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
|
|
|
|
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
|
|
|
|
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 Documentation/vm/numa_memory_policy.txt for a list of all available
|
|
memory allocation policy mode flags.
|
|
|
|
=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.
|
|
|
|
|
|
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
|