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 mod‐
ule 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 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 (EIN‐
VAL).
nel.org/pub/linux/utils/util-linux/.
util-linux August 1999 RAW(8)
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