bionic (8) raw.8.gz

Provided by: util-linux_2.31.1-0.4ubuntu3.7_amd64 bug

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, --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/.