RAW(8)									RAW(8)



NAME
       raw - bind a Linux raw character device

SYNOPSIS
       raw /dev/raw/raw  

       raw /dev/raw/raw /dev/

       raw -q /dev/raw/raw

       raw -qa

DESCRIPTION
       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
       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/ 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	kernels  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
       I/O.

       Because raw I/O involves direct hardware access to a processs  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.

OPTIONS
       -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
	      queried.

       -h     provides a usage summary.

BUGS
       The  Linux  dd  (1)  command  does not currently align its buffers cor
       rectly, and so cannot be used on raw devices.

       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!

AUTHOR
       Stephen Tweedie (sct@redhat.com)



Version 0.1			   Aug 1999				RAW(8)