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MD(4)                    BSD Kernel Interfaces Manual                    MD(4)

NAME
     md -- memory disk

SYNOPSIS
     device md

DESCRIPTION
     The md driver provides support for four kinds of memory backed virtual
     disks:

     malloc   Backing store is allocated using malloc(9).  Only one malloc-
              bucket is used, which means that all md devices with malloc
              backing must share the malloc-per-bucket-quota.  The exact size
              of this quota varies, in particular with the amount of RAM in
              the system.  The exact value can be determined with vmstat(8).

     preload  A file loaded by loader(8) with type 'md_image' is used for
              backing store.  For backwards compatibility the type 'mfs_root'
              is also recognized.  If the kernel is created with option
              MD_ROOT the first preloaded image found will become the root
              file system.

     vnode    A regular file is used as backing store.  This allows for mount-
              ing ISO images without the tedious detour over actual physical
              media.

     swap     Backing store is allocated from swap space.

     For more information, please see mdconfig(8).

SEE ALSO
     disklabel(5), disklabel(8), fdisk(8), loader(8), mdconfig(8), mdmfs(8),
     newfs(8), vmstat(8)

HISTORY
     The md driver first appeared in FreeBSD 4.0 as a cleaner replacement for
     the MFS functionality previously used in PicoBSD and in the FreeBSD
     installation process.

     The md driver did a hostile takeover of the vn(4) driver in FreeBSD 5.0.

AUTHORS
     The md driver was written by Poul-Henning Kamp <phkATFreeBSD.org>.

BSD                              March 9, 2001                             BSD