raw - bind a Linux raw character device
raw /dev/raw/raw<N> <major> <minor>
raw /dev/raw/raw<N> /dev/<blockdev>
raw -q /dev/raw/raw<N>
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
is used in two modes: it either sets raw device bindings, or it
queries existing bindings. When setting a raw device,
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
or as a path name /dev/<blockdev>
to an existing block device
The bindings already in existence can be queried with the -q
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.
- -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
- -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=
the blocksize needs to be a multiple of the sector size of the device (512
bytes usually), otherwise it will fail with "Invalid Argument"
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!
Stephen Tweedie (email@example.com)
The raw command is part of the util-linux package and is available from