.\"	$NetBSD: mount_tmpfs.8,v 1.18 2014/06/07 10:03:44 martin Exp $
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.\" by Julio M. Merino Vidal, developed as part of Google's Summer of Code
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.Dd June 7, 2014
.Dt MOUNT_TMPFS 8
.Os
.Sh NAME
.Nm mount_tmpfs
.Nd mount an efficient memory file system
.Sh SYNOPSIS
.Nm
.Op Fl g Ar group
.Op Fl m Ar mode
.Op Fl n Ar nodes
.Op Fl o Ar options
.Op Fl s Ar size
.Op Fl u Ar user
.Ar tmpfs
.Ar mount_point
.Sh DESCRIPTION
The
.Nm
command attaches an instance of the efficient memory file system to the
global file system namespace.
The
.Ar tmpfs
parameter only exists for compatibility with the other mount commands and
is ignored.
The directory specified by
.Ar mount_point
is converted to an absolute path before use and its attributes (owner,
group and mode) are inherited unless explicitly overridden by the options
described below.
.Pp
The following options are supported:
.Bl -tag -width XoXoptions
.It Fl g Ar group
Specifies the group name or GID of the root inode of the file system.
Defaults to the mount point's GID.
.It Fl m Ar mode
Specifies the mode (in octal notation) of the root inode of the file system.
Defaults to the mount point's mode.
.It Fl n Ar nodes
Specifies the maximum number of nodes available to the file system.
If not specified, the file system chooses a reasonable maximum given its
size at mount time, which can be limited with
.Fl s .
.It Fl o Ar options
Options are specified with a
.Fl o
flag followed by a comma-separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.It Fl s Ar size
Specifies the total file system size in bytes.
If zero is given (the default), the available amount of memory (including
main memory and swap space) will be used.
Note that some memory is always reserved for the system and cannot
be assigned to the file system.
The exact amount depends on the available memory and details of the
kernel memory usage, it might even change slightly during runtime.
.Ar Size
can alternatively be specified as a percentage of the available
system ram by using the notation
.Ar ram%n
where
.Ar n
is a number between 1 and 100.
Similarily it can be specified as a fraction of the available system
ram by using
.Ar ram/n
where
.Ar n
is the divisor.
(Using
.Ar ram%25
and
.Ar ram/4
will result in the same limit.)
.It Fl u Ar user
Specifies the user name or UID of the root inode of the file system.
Defaults to the mount point's UID.
.El
.Pp
Every option that accepts a numerical value as its argument can take a
trailing
.Sq b
to indicate bytes (the default), a
.Sq k
to indicate kilobytes, a
.Sq M
to indicate megabytes or a
.Sq G
to indicate gigabytes.
Note that both lowercase and uppercase forms of these letters are allowed.
.Sh EXAMPLES
The following command mounts a tmpfs instance over the
.Pa /tmp
directory, inheriting its owner, group and mode settings:
.Pp
.Dl # mount -t tmpfs tmpfs /tmp
.Pp
The following command mounts a tmpfs instance over the
.Pa /mnt
directory, setting a 20 megabytes limit in space, owned by the
.Sq joe
user and belonging to the
.Sq users
group, with a restricted 0700 mode:
.Pp
.Dl # mount -t tmpfs -o -s20M -o -ujoe -o -gusers -o -m0700 tmpfs /mnt
.Pp
See
.Pa /usr/share/examples/fstab/fstab.ramdisk
for some examples on how to add tmpfs entries to
.Pa /etc/fstab .
.Sh SEE ALSO
.Xr fstab 5 ,
.Xr mount 8
.Sh HISTORY
The
.Nm
utility first appeared in
.Nx 4.0 .
.Sh BUGS
File system meta-data is not pageable.
If there is not enough main memory to hold this information, the system may
become unstable or very unresponsive because it will not be able to allocate
required memory.
A malicious user could trigger this condition if he could create lots of
files inside a size-unbounded tmpfs file system.
Limiting the number of nodes per file system
.Pq Fl n
will prevent this; the default value for this setting is also often adjusted
to an adequate value to resolve this.