path: root/share/man/man5/tmpfs.5

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.Dd September 18, 2023
.Dt TMPFS 5
.Os
.Sh NAME
.Nm tmpfs
.Nd "in-memory file system"
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "options TMPFS"
.Ed
.Pp
Alternatively, to load the driver as a
module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
tmpfs_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
driver implements an in-memory, or
.Nm
file system.
The filesystem stores both file metadata and data in main memory.
This allows very fast and low latency accesses to the data.
The data is volatile.
An umount or system reboot invalidates it.
These properties make the filesystem's mounts suitable for fast
scratch storage, like
.Pa /tmp .
.Pp
If the system becomes low on memory and swap is configured
.Po see
.Xr swapon 8 Pc ,
the system can transfer file data to swap space, freeing memory
for other needs.
Metadata, including the directory content, is never swapped out by the
current implementation.
Keep this in mind when planning the mount limits, especially when expecting
to place many small files on a tmpfs mount.
.Pp
When
.Xr mmap 2
is used on a file from a tmpfs mount, the swap VM object managing the
file pages is used to implement mapping and avoid double-copying of
the file data.
This quirk causes process inspection tools, like
.Xr procstat 1 ,
to report anonymous memory mappings instead of file mappings.
.Sh OPTIONS
The following options are available when
mounting
.Nm
file systems:
.Bl -tag -width "maxfilesize"
.It Cm easize
Set the maximum memory size used by extended attributes in bytes.
The default is 16 megabytes.
.It Cm export
Accept the
.Cm export
option for compatibility with
.Xr nfsv4 4 .
This option does nothing.
.It Cm gid
Set the group ID of the root inode of the file system.
The default is the mount point's GID.
.It Cm inodes
Set the maximum number of nodes available to the file system.
If not specified, the file system chooses a reasonable maximum based on
the file system size, which can be limited with the
.Cm size
option.
.It Cm maxfilesize
Set the maximum file size in bytes.
The default is the maximum possible value.
.It Cm mode
Set the mode (in octal notation) of the root inode of the file system.
The default is the mount point's mode.
.It Cm nomtime
Disable the tracking of mtime updates caused by writes to the
shared mapped areas backed by
.Nm
files.
This option removes periodic scans,
which downgrade read-write-mapped pages to read-only to note the writes.
.It Cm nonc
Do not use namecache to resolve names to files for the created mount.
This saves memory, but currently might impair scalability for highly
used mounts on large machines.
.It Cm nosymfollow
Do not follow
.Xr symlink 7 Ap s
on the mounted file system.
.It Cm pgread
Enable pgcache read for the mount.
.It Cm size
Set the total file system size in bytes, unless suffixed
with one of k, m, g, t, or p, which denote byte, kilobyte,
megabyte, gigabyte, terabyte and petabyte respectively.
If zero (the default) or a value larger than SIZE_MAX - PAGE_SIZE
is given, the available amount of memory (including
main memory and swap space) will be used.
.It Cm uid
Set the user ID of the root inode of the file system.
The default is the mount point's UID.
.It Cm union
Refer to
.Xr mount 8 .
.El
.Sh EXAMPLES
Mount a
.Nm
memory file system:
.Pp
.Dl "mount -t tmpfs tmpfs /tmp"
.Pp
Configure a
.Nm
mount via
.Xr fstab 5 :
.Bd -literal -offset indent
tmpfs /tmp tmpfs rw 0 0
.Ed
.Sh SEE ALSO
.Xr procstat 1 ,
.Xr mmap 2 ,
.Xr nmount 2 ,
.Xr unmount 2 ,
.Xr fstab 5 ,
.Xr mdmfs 8 ,
.Xr mount 8 ,
.Xr swapinfo 8 ,
.Xr swapon 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 7.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
kernel implementation was written by
.An Julio M. Merino Vidal Aq Mt jmmv@NetBSD.org
as a Google Summer of Code project.
.Pp
.An Rohit Jalan
and others ported it from
.Nx
to
.Fx .
.Pp
This manual page was written by
.An Xin LI Aq Mt delphij@FreeBSD.org .