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

       gpart(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>.

Debian                                          October 30, 2007                                           MD(4)