Messages in this thread Patch in this message | | | From | Christoph Rohland <> | Subject | [Patch] tmpfs documentation | Date | 01 Dec 2001 18:07:46 +0100 |
| |
Hi Linus and Marcelo,
The appended patch adds and reorganizes the documentation for tmpfs.
It is against 2.4.16 but should apply to 2.5 also.
Please apply to both versions
Christoph
diff -uNr 2.4.16/Documentation/Configure.help m2.4.16/Documentation/Configure.help --- 2.4.16/Documentation/Configure.help Mon Nov 26 16:21:44 2001 +++ m2.4.16/Documentation/Configure.help Sat Dec 1 17:58:42 2001 @@ -13839,30 +13839,11 @@ CONFIG_TMPFS Tmpfs is a file system which keeps all files in virtual memory. - In contrast to RAM disks, which get allocated a fixed amount of - physical RAM, tmpfs grows and shrinks to accommodate the files it - contains and is able to swap unneeded pages out to swap space. + Everything in tmpfs is temporary in the sense that no files will be + created on your hard drive. If you reboot, everything in tmpfs will + be lost. - Everything is "virtual" in the sense that no files will be created - on your hard drive; if you reboot, everything in tmpfs will be - lost. - - You should mount the file system somewhere to be able to use - POSIX shared memory. Adding the following line to /etc/fstab should - take care of things: - - 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). - - You can set limits for the number of blocks and inodes used by the - file system with the mount options "size", "nr_blocks" and - "nr_inodes". These parameters accept a suffix k, m or g for kilo, - mega and giga and can be changed on remount. - - The initial permissions of the root directory can be set with the - mount option "mode". + See <file:Documentation/filesystems/tmpfs.txt> for details Simple RAM-based file system support CONFIG_RAMFS diff -uNr 2.4.16/Documentation/filesystems/tmpfs.txt m2.4.16/Documentation/filesystems/tmpfs.txt --- 2.4.16/Documentation/filesystems/tmpfs.txt Thu Jan 1 01:00:00 1970 +++ m2.4.16/Documentation/filesystems/tmpfs.txt Sat Dec 1 18:02:26 2001 @@ -0,0 +1,96 @@ +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 reboot, everything in tmpfs will be +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. + +The initial permissions of the root directory can be set with the +mount option "mode". Later on you can change the permissions of the +root directory with chmod. + +So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=0700 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 autatically 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |