RAW(8)                                System Administration                                RAW(8)

       raw - bind a Linux raw character device

       raw /dev/raw/raw  

       raw /dev/raw/raw /dev/

       raw -q /dev/raw/raw

       raw -qa

       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 bind‐
       ings.  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 speci‐
       fied 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, 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 per‐
       forming  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.

       -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.

       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 delib‐
       erate, but is regarded either a bug or a feature depending on who you ask!

       Stephen Tweedie (sct@redhat.com)

       The  raw  command  is  part of the util-linux package and is available from ftp://ftp.ker‐

util-linux                                 August 1999                                     RAW(8)


Designed by SanjuD(@ngineerbabu)