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Documentation / filesystems / tmpfs.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	Tmpfs is a file system which keeps all files in virtual memory.
2	
3	
4	Everything in tmpfs is temporary in the sense that no files will be
5	created on your hard drive. If you unmount a tmpfs instance,
6	everything stored therein is lost.
7	
8	tmpfs puts everything into the kernel internal caches and grows and
9	shrinks to accommodate the files it contains and is able to swap
10	unneeded pages out to swap space. It has maximum size limits which can
11	be adjusted on the fly via 'mount -o remount ...'
12	
13	If you compare it to ramfs (which was the template to create tmpfs)
14	you gain swapping and limit checking. Another similar thing is the RAM
15	disk (/dev/ram*), which simulates a fixed size hard disk in physical
16	RAM, where you have to create an ordinary filesystem on top. Ramdisks
17	cannot swap and you do not have the possibility to resize them. 
18	
19	Since tmpfs lives completely in the page cache and on swap, all tmpfs
20	pages currently in memory will show up as cached. It will not show up
21	as shared or something like that. Further on you can check the actual
22	RAM+swap use of a tmpfs instance with df(1) and du(1).
23	
24	
25	tmpfs has the following uses:
26	
27	1) There is always a kernel internal mount which you will not see at
28	   all. This is used for shared anonymous mappings and SYSV shared
29	   memory. 
30	
31	   This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
32	   set, the user visible part of tmpfs is not build. But the internal
33	   mechanisms are always present.
34	
35	2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
36	   POSIX shared memory (shm_open, shm_unlink). Adding the following
37	   line to /etc/fstab should take care of this:
38	
39		tmpfs	/dev/shm	tmpfs	defaults	0 0
40	
41	   Remember to create the directory that you intend to mount tmpfs on
42	   if necessary.
43	
44	   This mount is _not_ needed for SYSV shared memory. The internal
45	   mount is used for that. (In the 2.3 kernel versions it was
46	   necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
47	   shared memory)
48	
49	3) Some people (including me) find it very convenient to mount it
50	   e.g. on /tmp and /var/tmp and have a big swap partition. And now
51	   loop mounts of tmpfs files do work, so mkinitrd shipped by most
52	   distributions should succeed with a tmpfs /tmp.
53	
54	4) And probably a lot more I do not know about :-)
55	
56	
57	tmpfs has three mount options for sizing:
58	
59	size:      The limit of allocated bytes for this tmpfs instance. The 
60	           default is half of your physical RAM without swap. If you
61	           oversize your tmpfs instances the machine will deadlock
62	           since the OOM handler will not be able to free that memory.
63	nr_blocks: The same as size, but in blocks of PAGE_CACHE_SIZE.
64	nr_inodes: The maximum number of inodes for this instance. The default
65	           is half of the number of your physical RAM pages, or (on a
66	           machine with highmem) the number of lowmem RAM pages,
67	           whichever is the lower.
68	
69	These parameters accept a suffix k, m or g for kilo, mega and giga and
70	can be changed on remount.  The size parameter also accepts a suffix %
71	to limit this tmpfs instance to that percentage of your physical RAM:
72	the default, when neither size nor nr_blocks is specified, is size=50%
73	
74	If nr_blocks=0 (or size=0), blocks will not be limited in that instance;
75	if nr_inodes=0, inodes will not be limited.  It is generally unwise to
76	mount with such options, since it allows any user with write access to
77	use up all the memory on the machine; but enhances the scalability of
78	that instance in a system with many cpus making intensive use of it.
79	
80	
81	tmpfs has a mount option to set the NUMA memory allocation policy for
82	all files in that instance (if CONFIG_NUMA is enabled) - which can be
83	adjusted on the fly via 'mount -o remount ...'
84	
85	mpol=default             use the process allocation policy
86	                         (see set_mempolicy(2))
87	mpol=prefer:Node         prefers to allocate memory from the given Node
88	mpol=bind:NodeList       allocates memory only from nodes in NodeList
89	mpol=interleave          prefers to allocate from each node in turn
90	mpol=interleave:NodeList allocates from each node of NodeList in turn
91	mpol=local		 prefers to allocate memory from the local node
92	
93	NodeList format is a comma-separated list of decimal numbers and ranges,
94	a range being two hyphen-separated decimal numbers, the smallest and
95	largest node numbers in the range.  For example, mpol=bind:0-3,5,7,9-15
96	
97	A memory policy with a valid NodeList will be saved, as specified, for
98	use at file creation time.  When a task allocates a file in the file
99	system, the mount option memory policy will be applied with a NodeList,
100	if any, modified by the calling task's cpuset constraints
101	[See Documentation/cgroups/cpusets.txt] and any optional flags, listed
102	below.  If the resulting NodeLists is the empty set, the effective memory
103	policy for the file will revert to "default" policy.
104	
105	NUMA memory allocation policies have optional flags that can be used in
106	conjunction with their modes.  These optional flags can be specified
107	when tmpfs is mounted by appending them to the mode before the NodeList.
108	See Documentation/vm/numa_memory_policy.txt for a list of all available
109	memory allocation policy mode flags and their effect on memory policy.
110	
111		=static		is equivalent to	MPOL_F_STATIC_NODES
112		=relative	is equivalent to	MPOL_F_RELATIVE_NODES
113	
114	For example, mpol=bind=static:NodeList, is the equivalent of an
115	allocation policy of MPOL_BIND | MPOL_F_STATIC_NODES.
116	
117	Note that trying to mount a tmpfs with an mpol option will fail if the
118	running kernel does not support NUMA; and will fail if its nodelist
119	specifies a node which is not online.  If your system relies on that
120	tmpfs being mounted, but from time to time runs a kernel built without
121	NUMA capability (perhaps a safe recovery kernel), or with fewer nodes
122	online, then it is advisable to omit the mpol option from automatic
123	mount options.  It can be added later, when the tmpfs is already mounted
124	on MountPoint, by 'mount -o remount,mpol=Policy:NodeList MountPoint'.
125	
126	
127	To specify the initial root directory you can use the following mount
128	options:
129	
130	mode:	The permissions as an octal number
131	uid:	The user id 
132	gid:	The group id
133	
134	These options do not have any effect on remount. You can change these
135	parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
136	
137	
138	So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
139	will give you tmpfs instance on /mytmpfs which can allocate 10GB
140	RAM/SWAP in 10240 inodes and it is only accessible by root.
141	
142	
143	Author:
144	   Christoph Rohland <cr@sap.com>, 1.12.01
145	Updated:
146	   Hugh Dickins, 4 June 2007
147	Updated:
148	   KOSAKI Motohiro, 16 Mar 2010
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