bionic (4) md.4freebsd.gz

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>.