raw

RAW(8)								       RAW(8)



NAME
       raw - bind a Linux raw character device

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

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

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

       raw -qa

WARNING
       Although	 Linux	includes  support  for	rawio, it is now a deprecated
       interface. If your  application	performs  device  access  using	 this
       interface,  Red	Hat encourages you to modify your application to open
       the block device with the O_DIRECT  flag.  The  rawio  interface	 will
       exist  for  the life of Red Hat Enterprise Linux 4, but is a candidate
       for removal from future releases.

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<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 I/O.

       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
       exception that the "raw" command itself is omitted.

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  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
       (EINVAL).


       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)