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

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, 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 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.

OPTIONS

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

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

       -h     Provide a usage summary.

BUGS

       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 (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)

AVAILABILITY

       The   raw   command   is   part   of   the   util-linux  package  and  is  available  from
       ftp://ftp.kernel.org/pub/linux/utils/util-linux/.