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RAW(8)									RAW(8)

       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,
       with 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.

       Use  the /etc/sysconfig/rawdevices file to define the set of raw device
       mappings automatically created during the system startup sequence.  The
       format of the file is the same used in the command line with the excep-
       tion that the "raw" command itself is omitted.

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

       -a     With  -q	,  specifies  that  all	 bound	raw  devices should be

       -h     provides a usage summary.

       The Linux dd (1) command should be  used	 without  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)

Version 0.1			   Aug 1999				RAW(8)
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