raw

RAW(8)                       System Administration                      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,
       which is used either with a raw device filename to query that  one  de-
       vice, 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  en-
       tirely:  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, --query
              Set  query  mode.  raw will query an existing binding instead of
              setting a new one.

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

       -h, --help
              Display help text and exit.

       -V, --version
              Display version information and exit.

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!

NOTES
       Rather  than  using  raw devices applications should prefer open(2) de-
       vices, such as /dev/sda1, with the O_DIRECT flag.

AUTHOR
       Stephen Tweedie (sct@redhat.com)

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

util-linux                        August 1999                           RAW(8)