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mdconfig - configure and enable memory disks



      mdconfig - configure and enable memory disks


      mdconfig -a -t type [-n] [-o [no]option] ... [-f file] [-s size]
               [-S sectorsize] [-u unit] [-x sectors/track] [-y heads/cyl]
      mdconfig -d -u unit
      mdconfig -l [-n] [-u unit]


      The mdconfig utility configures and enables md(4) devices.
      Options indicate an action to be performed:
      -a      Attach a memory disk.  This will configure and attach a memory
              disk with the parameters specified and attach it to the system.
      -d      Detach a memory disk from the system and release all resources.
      -t type
              Select the type of the memory disk.
              malloc   Storage for this type of memory disk is allocated with
                       malloc(9).  This limits the size to the malloc bucket
                       limit in the kernel.  If the -o reserve option is not
                       set, creating and filling a large malloc-backed memory
                       disk is a very easy way to panic a system.
              vnode    A file specified with -f file becomes the backingstore
                       for this memory disk.
              swap     Storage for this type of memory disk 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 gener‐
                       ally preferable over malloc backing.
      -f file
              Filename to use for the vnode type memory disk. Options -a and -t
              vnode are implied if not specified.
      -l      List configured devices.  If given with -u, display details about
              that particular device.
      -n      When printing md device names, print only the unit number without
              the md prefix.
      -s size
              Size of the memory disk.  Size is the number of 512 byte sectors
              unless suffixed with a b, k, m, g, or t which denotes byte, kilo‐
              byte, megabyte, gigabyte and terabyte respectively. Options -a
              and -t swap are implied if not specified.
      -S sectorsize
              Sectorsize to use for malloc backed device.
      -x sectors/track
              See the description of the -y option below.
      -y heads/cylinder
              For malloc or vnode backed devices, the -x and -y options can be
              used to specify a synthetic geometry.  This is useful for con‐
              structing bootable images for later download to other devices.
      -o [no]option
              Set or reset options.
                      For vnode backed devices: avoid IO_SYNC for increased
                      performance but at the risk of deadlocking the entire
                      Allocate and reserve all needed storage from the start,
                      rather than as needed.
                      Enable clustering on this disk.
                      Enable/Disable compression features to reduce memory
                      Disable/Enable extra sanity checks to prevent the user
                      from doing something that might adversely affect the sys‐
                      Enable/Disable readonly mode.
      -u unit
              Request a specific unit number for the md(4) device instead of
              automatic allocation.


      To create a 4 megabyte malloc(9) backed memory disk.  The name of the
      allocated unit will be output on stdout like “md3”:
            mdconfig -a -t malloc -s 4m
      To create a disk named /dev/md4 with /tmp/boot.flp as backing:
            mdconfig -a -t vnode -f /tmp/boot.flp -u 4
      To detach and free all resources used by /dev/md4:
            mdconfig -d -u 4
      To create and mount a 128MByte swap backed file system on /tmp:
            mdconfig -a -t swap -s 128M -u 10
            newfs -U /dev/md10
            mount /dev/md10 /tmp
            chmod 1777 /tmp
      To create a 5MB file-backed disk ( --a and --t vnode are implied):
            dd if=/dev/zero of=somebackingfile bs=1k count=5k
            mdconfig -f somebackingfile -u 0
            bsdlabel -w md0 auto
            newfs md0c
            mount /dev/md0c /mnt
      To mount an ISO 9660 CD image file:
            mdconfig -a -t vnode -u 10 -f cdimage.iso
            mount_cd9660 /dev/md10 /mnt
      md(4), bsdlabel(8), fdisk(8), mdmfs(8), malloc(9)


      The mdconfig utility first appeared in FreeBSD 5.0 as a cleaner replace‐
      ment for the vn(4) and vnconfig(8) combo.


      The mdconfig utility was written by Poul-Henning Kamp 〈〉.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.