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MD(4)                 DragonFly Kernel Interfaces Manual                 MD(4)


NAME

     md -- memory disk


SYNOPSIS

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

           device md

     Alternatively, to load the driver as a module at boot time, place the
     following line in /boot/loader.conf:

           md_load="YES"

     To specify the default maximum size of kmalloc(9) backed disks:
     options MD_NSECT=20000


DESCRIPTION

     The md driver provides support for two kinds of memory backed virtual
     disks: kmalloc(9) backed and pre-loaded module backed.

     The kmalloc(9) backed disks are severely limited in size by constraints
     imposed by kmalloc(9).  Specifically, only one malloc bucket is used,
     which means that all md devices with kmalloc(9) backing must share the
     malloc per-bucket quota.  The exact size of this quota varies, in partic-
     ular with the amount of RAM in the system.  The exact value can be deter-
     mined with vmstat(8).

     A sector filled with identical bytes does not consume storage, and there-
     fore the storage allocated by a md disk can be freed with a command such
     as this:

           dd if=/dev/zero of=/dev/md567

     The default maximum size of a md disk backed by kmalloc(9) is 20,000 sec-
     tors of 512 bytes each.  This can be changed with the kernel option
     MD_NSECT.

     At boot time the md driver will search for pre-loaded modules of type
     `md_image' and instantiate a md device for each of these modules.  The
     type `mfs_root' is also allowed for backward compatibility.  These
     devices are backed by the RAM reserved by the loader(8), and as such not
     limited by the kmalloc(9) size constraints.  However, this also means
     that the storage cannot be released.  For the same reason, the
     ``identical byte'' detection is not applicable.

     The md driver uses the ``almost-clone'' convention, whereby opening
     device number N creates device instance number N+1.

     The vmstat(8) utility can be used to monitor memory usage by kmalloc(9)
     backed md disks, but not those backed by pre-loaded modules.


EXAMPLES

     To mount a kmalloc(9) backed md device on /tmp:

     if [ -e /dev/md0 ]; then
             newfs /dev/md0 && \
             mount /dev/md0 /tmp && \
             chmod 1777 /tmp
     fi


SEE ALSO

     disklabel(5), disklabel(8), fdisk(8), loader(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.


AUTHORS

     The md driver was written by Poul-Henning Kamp <phk@FreeBSD.org>.

DragonFly 3.5                 September 28, 2009                 DragonFly 3.5

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