raw - bind a Linux raw character device


   raw /dev/raw/raw<N> <major> <minor>

   raw /dev/raw/raw<N> /dev/<blockdev>

   raw -q /dev/raw/raw<N>

   raw -qa


   raw  is  used  to  bind a Linux raw character device to a block device.
   Any block device may be used: at the time of binding, the device driver
   does  not  even  have to be accessible (it may be loaded on demand as a
   kernel module later).

   raw is used in two modes: it either sets raw  device  bindings,  or  it
   queries  existing bindings.  When setting a raw device, /dev/raw/raw<N>
   is the device name of an existing raw device node  in  the  filesystem.
   The  block device to which it is to be bound can be specified either in
   terms of its major  and  minor  device  numbers,  or  as  a  path  name
   /dev/<blockdev> to an existing block device file.

   The  bindings  already  in existence can be queried with the -q option,
   which is used either with a raw  device  filename  to  query  that  one
   device, or with the -a option to query all bound raw devices.

   Unbinding can be done by specifying major and minor 0.

   Once  bound  to  a  block  device, a raw device can be opened, read and
   written, just like the block device it is bound to.  However,  the  raw
   device  does  not behave exactly like the block device.  In particular,
   access to the raw device  bypasses  the  kernel's  block  buffer  cache
   entirely: all I/O is done directly to and from the address space of the
   process performing the I/O.  If the underlying block device driver  can
   support  DMA,  then  no data copying at all is required to complete the

   Because raw I/O involves direct hardware access to a process's  memory,
   a  few extra restrictions must be observed.  All I/Os must be correctly
   aligned in memory and on disk: they must start at a  sector  offset  on
   disk, they must be an exact number of sectors long, and the data buffer
   in virtual memory must also be aligned to  a  multiple  of  the  sector
   size.  The sector size is 512 bytes for most devices.


   -q, --query
          Set  query  mode.  raw will query an existing binding instead of
          setting a new one.

   -a, --all
          With -q , specify that all bound raw devices should be queried.

   -h, --help
          Display help text and exit.

   -V, --version
          Display version information and exit.


   The Linux dd(1) command should be used without the bs= option,  or  the
   blocksize  needs to be a multiple of the sector size of the device (512
   bytes usually), otherwise it will fail with "Invalid Argument" messages

   Raw  I/O  devices  do not maintain cache coherency with the Linux block
   device buffer cache.  If you use raw I/O to overwrite data  already  in
   the  buffer  cache,  the  buffer cache will no longer correspond to the
   contents of the actual storage device underneath.  This is  deliberate,
   but is regarded either a bug or a feature depending on who you ask!


   Stephen Tweedie (sct@redhat.com)


   The raw command is part of the util-linux package and is available from


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