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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
              mounting ISO images without the tedious detour over actual
              physical media.

     swap     Backing store is allocated from buffer memory.  Pages get pushed
              out to the swap when the system is under memory pressure,
              otherwise they stay in the operating memory.  Using swap backing
              is generally preferable over malloc backing.

     For more information, please see mdconfig(8).

EXAMPLES

     To create a kernel with a ramdisk or MD file system, your kernel config
     needs the following options:

           options         MD_ROOT                 # MD is a potential root device
           options         MD_ROOT_SIZE=8192       # 8MB ram disk
           makeoptions     MFS_IMAGE=/h/foo/ARM-MD
           options         ROOTDEVNAME=\"ufs:md0\"

     The image in /h/foo/ARM-MD will be loaded as the initial image each boot.
     To create the image to use, please follow the steps to create a file-
     backed disk found in the mdconfig(8) man page.  Other tools will also
     create these images, such as NanoBSD.

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 <phk@FreeBSD.org>.