patch-2.4.17 linux/Documentation/filesystems/tmpfs.txt

• Lines: 103
• Date: Fri Dec 21 16:40:32 2001
• Orig file: linux-2.4.16/Documentation/filesystems/tmpfs.txt
• Orig date: Thu Jan 1 00:00:00 1970

diff -Naur -X /home/marcelo/lib/dontdiff linux-2.4.16/Documentation/filesystems/tmpfs.txt linux/Documentation/filesystems/tmpfs.txt
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+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 (/dev/shm is automagically created if you use devfs).
+
+   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. But be
+   aware: loop mounts of tmpfs files do not work due to the internal
+   design. So mkinitrd shipped by most distributions will fail with a
+   tmpfs /tmp.
+
+4) And probably a lot more I do not know about :-)
+
+
+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 '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.
+
+
+TODOs:
+
+1) give the size option a percent semantic: If you give a mount option
+   size=50% the tmpfs instance should be able to grow to 50 percent of
+   RAM + swap. So the instance should adapt automatically if you add
+   or remove swap space.
+2) loop mounts: This is difficult since loop.c relies on the readpage
+   operation. This operation gets a page from the caller to be filled
+   with the content of the file at that position. But tmpfs always has
+   the page and thus cannot copy the content to the given page. So it
+   cannot provide this operation. The VM had to be changed seriously
+   to achieve this.
+3) Show the number of tmpfs RAM pages. (As shared?)
+
+Author:
+   Christoph Rohland <cr@sap.com>, 1.12.01