rawdevices

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

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.

       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 excep-
       tion 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 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)