raw(8) — Linux manual page


RAW(8)                      System Administration                     RAW(8)

NAME         top

       raw - bind a Linux raw character device

SYNOPSIS         top

       raw /dev/raw/raw<N> <major> <minor>

       raw /dev/raw/raw<N> /dev/<blockdev>

       raw -q /dev/raw/raw<N>

       raw -qa

DESCRIPTION         top

       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         top

       -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.

NOTES         top

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

BUGS         top

       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 as either a bug or a feature, depending
       on who you ask!

AUTHORS         top

       Stephen Tweedie (sct@redhat.com)

AVAILABILITY         top

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

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.  If you have a
       bug report for this manual page, send it to
       util-linux@vger.kernel.org.  This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-15.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

util-linux                       August 1999                          RAW(8)

Pages that refer to this page: creat(2)open(2)openat(2)