mirror of https://github.com/mkerrisk/man-pages
130 lines
3.5 KiB
Groff
130 lines
3.5 KiB
Groff
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.\" Copyright (c) 2016 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH TMPFS 5 2016-11-07 "Linux" "Linux Programmer's Manual"
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.SH NAME
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tmpfs \- a virtual memory filesystem
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.SH DESCRIPTION
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The
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.B tmpfs
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facility allows the creation of filesystems whose contents reside
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in virtual memory.
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Since the files on such filesystems typically reside in RAM,
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file access is extremely fast.
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The filesystem is automatically created when mounting
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a filesystem with the type
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.BR tmpfs
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via a command such as the following:
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$ sudo mount \-t tmpfs -o size=10M tmpfs /mnt/mytmpfs
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A
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.B tmpfs
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filesystem has the following properties:
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.IP * 3
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The filesystem can employ swap space when physical memory pressure
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demands it.
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.IP *
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The
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.I size
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option can be used to specify an upper limit on the size of the filesystem.
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(The default size is half of the available RAM size.)
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The filesystem consumes only as much physical memory and swap space
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as is required to store the current contents of the filesystem.
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.IP *
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During a remount operation
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.RI ( "mount\ \-o\ remount" ),
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the filesystem size can be changed
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(without losing the existing contents of the filesystem).
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.PP
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If a
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.B tmpfs
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filesystem is unmounted, its contents are discarded (lost).
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.SH VERSIONS
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The
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.B tmpfs
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facility was added in Linux 2.4, as a successor to the older
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.B ramfs
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facility, which did not provide limit checking or
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allow for the use of swap space.
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.SH NOTES
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For a description of the mount options that may be employed when mounting a
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.B tmpfs
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filesystem, see
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.BR mount (8).
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In order to for user-space tools and applications to create
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.B tmpfs
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filesystems, the kernel must be configured with the
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.B CONFIG_TMPFS
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option.
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An internal shared memory filesystem is used for
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System V shared memory
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.BR (shmget (2))
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and shared anonymous mappings
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.RB ( mmap (2)
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with the
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.B MAP_SHARED
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and
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.BR MAP_ANONYMOUS
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flags).
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This filesystem is available regardless of whether
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the kernel was configured with the
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.B CONFIG_TMPFS
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option.
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A
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.B tmpfs
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filesystem mounted at
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.IR /dev/shm
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as used for the implementation of POSIX shared memory
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.RB ( shm_overview (7))
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and POSIX semaphores
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.RB ( sem_overview (7)).
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The amount of memory consumed by all
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.B tmpfs
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filesystems is shown in the
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.I Shmem
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field of
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.IR /proc/meminfo
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and in the
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.I shared
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field displayed by
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.BR free (1).
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The
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.B tmpfs
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facility was formerly called
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.BR shmfs .
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.SH SEE ALSO
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.BR df (1),
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.BR du (1),
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.BR mount (8)
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.PP
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The kernel source file
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.IR Documentation/filesystems/tmpfs.txt .
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