1.\" 2.\" Copyright (c) 1990, Sun Microsystems, Inc. 3.\" Copyright 2021 Oxide Computer Company 4.\" 5.\" The contents of this file are subject to the terms of the 6.\" Common Development and Distribution License (the "License"). 7.\" You may not use this file except in compliance with the License. 8.\" 9.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE 10.\" or http://www.opensolaris.org/os/licensing. 11.\" See the License for the specific language governing permissions 12.\" and limitations under the License. 13.\" 14.\" When distributing Covered Code, include this CDDL HEADER in each 15.\" file and include the License file at usr/src/OPENSOLARIS.LICENSE. 16.\" If applicable, add the following below this CDDL HEADER, with the 17.\" fields enclosed by brackets "[]" replaced with your own identifying 18.\" information: Portions Copyright [yyyy] [name of copyright owner] 19.\" 20.Dd October 9, 1990 21.Dt TMPFS 4FS 22.Os 23.Sh NAME 24.Nm tmpfs 25.Nd memory based file system 26.Sh SYNOPSIS 27.In sys/mount.h 28.Fo mount 29.Fa "const char *special" 30.Fa "const char *directory" 31.Fa IMS_DATA 32.Fa \(dqtmpfs\(dq 33.Fa NULL 34.Fa 0 35.Fc 36.Sh DESCRIPTION 37.Nm 38is a memory based file system which uses kernel resources relating to the VM 39system and page cache as a file system. 40Once mounted, a 41.Nm 42file system provides standard file operations and semantics. 43.Nm 44is so named because files and directories are not preserved across 45reboot or unmounts, all files residing on a 46.Nm 47file system that is unmounted will be lost. 48.Pp 49.Nm 50file systems can be mounted with the command: 51.Bd -literal -offset indent 52mount -F tmpfs swap directory 53.Ed 54.Pp 55Alternatively, to mount a 56.Nm 57file system on 58.Pa /tmp 59at multi-user startup time 60.Pq maximizing possible performance improvements , 61add the following line to 62.Pa /etc/vfstab : 63.Bd -literal -offset indent 64swap \(mi/tmp tmpfs \(mi yes \(mi 65.Ed 66.Pp 67.Nm 68is designed as a performance enhancement which is achieved by caching the writes 69to files residing on a 70.Nm 71file system. 72Performance improvements are most noticeable when a large number of short lived 73files are written and accessed on a 74.Nm 75file system. 76Large compilations with 77.Nm 78mounted on 79.Pa /tmp 80are a good example of this. 81.Pp 82Users of 83.Nm 84should be aware of some constraints involved in mounting a 85.Nm 86file system. 87The resources used by 88.Nm 89are the same as those used when commands are executed 90.Pq for example, swap space allocation . 91This means that large sized 92.Nm 93files can affect the amount of space left over for programs to execute. 94Likewise, programs requiring large amounts of memory use up the space available 95to 96.Nm 97Users running into this constraint 98.Po 99for example, running out of space on 100.Nm 101.Pc 102can allocate more swap space by using the 103.Xr swap 8 104command. 105.Pp 106Another constraint is that the number of files available in a 107.Nm 108file system is calculated based on the physical memory of the machine and not 109the size of the swap device/partition. 110If you have too many files, 111.Nm 112will print a warning message and you will be unable to create new files. 113You cannot increase this limit by adding swap space. 114.Pp 115Normal file system writes are scheduled to be written to a permanent storage 116medium along with all control information associated with the file 117.Pq for example, modification time, file permissions . 118.Nm 119control information resides only in memory and never needs to be written to 120permanent storage. 121File data remains in core until memory demands are sufficient to cause pages 122associated with 123.Nm 124to be reused at which time they are copied out to swap. 125.Pp 126An additional mount option can be specified to control the size of an individual 127.Nm 128file system. 129See 130.Xr mount_tmpfs 8 131for more details. 132.Sh DIAGNOSTICS 133If 134.Nm 135runs out of space, one of the following messages will display in the console. 136.Bl -tag -width Ds 137.It "directory: File system full, swap space limit exceeded" 138This message appears because a page could not be allocated while writing to a 139file. 140This can occur if 141.Nm 142is attempting to write more than it is allowed, or if currently executing 143programs are using a lot of memory. 144To make more space available, remove unnecessary files, exit from some programs, 145or allocate more swap space using 146.Xr swap 8 . 147.It "directory: File system full, memory allocation failed" 148.Nm 149ran out of physical memory while attempting to create a new file or 150directory. 151Remove unnecessary files or directories or install more physical memory. 152.El 153.Sh SEE ALSO 154.Xr mmap 2 , 155.Xr mount 2 , 156.Xr umount 2 , 157.Xr vfstab 5 , 158.Xr df 8 , 159.Xr mount 8 , 160.Xr mount_tmpfs 8 , 161.Xr swap 8 162.Sh WARNINGS 163Files and directories on a 164.Nm 165file system are not preserved across reboots or unmounts. 166Command scripts or programs which count on this will not work as expected. 167.Sh NOTES 168Compilers do not necessarily use 169/tmp 170to write intermediate files therefore missing some significant performance 171benefits. 172This can be remedied by setting the environment variable 173.Ev TMPDIR to 174.Pa /tmp . 175Compilers use 176the value in this environment variable as the name of the directory to store 177intermediate files. 178.Pp 179.Sy swap 180to a 181.Nm 182file is not supported. 183.Pp 184.Xr df 8 185output is of limited accuracy since a 186.Nm 187file system size 188is not static and the space available to 189.Nm 190is dependent on the swap space demands of the entire system. 191