TMPFS(5) Linux Programmer's Manual TMPFS(5)
tmpfs - a virtual memory filesystem
The tmpfs facility allows the creation of filesystems whose contents reside in virtual memory. Since the files on such filesystems typically reside in RAM, file access is extremely fast. The filesystem is automatically created when mounting a filesystem with the type tmpfs via a command such as the following: $ sudo mount -t tmpfs -o size=10M tmpfs /mnt/mytmpfs A tmpfs filesystem has the following properties: * The filesystem can employ swap space when physical memory pressure demands it. * The size option can be used to specify an upper limit on the size of the filesystem. (The default size is half of the available RAM size.) The filesystem consumes only as much physical memory and swap space as is required to store the current contents of the filesystem. * During a remount operation (mount -o remount), the filesystem size can be changed (without losing the existing contents of the filesystem). If a tmpfs filesystem is unmounted, its contents are discarded (lost).
The tmpfs facility was added in Linux 2.4, as a successor to the older ramfs facility, which did not provide limit checking or allow for the use of swap space.
For a description of the mount options that may be employed when mounting a tmpfs filesystem, see mount(8). In order to for user-space tools and applications to create tmpfs filesystems, the kernel must be configured with the CONFIG_TMPFS option. An internal shared memory filesystem is used for System V shared memory (shmget(2)) and shared anonymous mappings (mmap(2) with the MAP_SHARED and MAP_ANONYMOUS flags). This filesystem is available regardless of whether the kernel was configured with the CONFIG_TMPFS option. A tmpfs filesystem mounted at /dev/shm as used for the implementation of POSIX shared memory (shm_overview(7)) and POSIX semaphores (sem_overview(7)). The amount of memory consumed by all tmpfs filesystems is shown in the Shmem field of /proc/meminfo and in the shared field displayed by free(1). The tmpfs facility was formerly called shmfs.
df(1), du(1), mount(8) The kernel source file Documentation/filesystems/tmpfs.txt.
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