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
       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  cor-
       rectly  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) devices, 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)