Ram Disk

About

A "RAM Disk" can give a major performance boost under the right circumstances, namely speedy access to volatile data that is write-heavy and doesn't need to persist across reboots.

Linux

tmpfs is an incarnation of RAM disk on Linux.

Current Linux distros typically mount one or more tmpfs automatically.

Debian/Ubuntu

$ df -k /run
Filesystem 1K-blocks Used Available Use% Mounted on
tmpfs 401168 1236 399932 1% /run

$ mount | grep tmpfs
none on /sys/fs/cgroup type tmpfs (rw)
udev on /dev type devtmpfs (rw,mode=0755)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)

RedHat/CentOS

# uname -a
Linux centos5.mdf2.loc 2.6.18-371.3.1.el5 #1 SMP Thu Dec 5 12:47:01 EST 2013 i686 i686 i386 GNU/Linux

df -k /dev/shm

Filesystem 1K-blocks Used Available Use% Mounted on
tmpfs 257428 0 257428 0% /dev/shm

mount | grep tmpfs

tmpfs on /dev/shm type tmpfs (rw)

Options

Here's an excerpt from mount(8) man page.

Mount options for tmpfs
size=nbytes
Override default maximum size of the filesystem. The size is
given in bytes, and rounded up to entire pages. The default is
half of the memory. The size parameter also accepts a suffix  %
to limit this tmpfs instance to that percentage of your physical
RAM: the default, when neither size nor nr_blocks is specified,
is size=50%

   nr\_blocks=
          The same as size, but in blocks of PAGE\_CACHE\_SIZE

   nr\_inodes=
          The  maximum  number of inodes for this instance. The default is
          half of the number of your physical RAM pages, or (on a  machine
          with  highmem)  the number of lowmem RAM pages, whichever is the
          lower.

   The tmpfs mount options for sizing ( size,  nr\_blocks,  and  nr\_inodes)
   accept  a  suffix k, m or g for Ki, Mi, Gi (binary kilo, mega and giga)
   and can be changed on remount.

   mode=  Set initial permissions of the root directory.

   uid=   The user id.

   gid=   The group id.

FreeBSD

Under FreeBSD a ram disk can be created using mdconfig.

See http://ryanbowlby.com/blog/2009/09/30/freebsd-ramdisk-mdconfig/ for a nice write-up.

Options

See mdconfig(8) and md(4) man pages for options.