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>
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 bind-
ings. 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 speci-
fied either in terms of its major and minor device numbers, or as a path name /dev/<block-
dev> to an existing block device file.
The bindings already in existence can be queried with the -q option, with is used either
with a raw device filename to query that one device, or with the -a option to query all
bound raw devices.
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 per-
forming 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.
Use the /etc/sysconfig/rawdevices file to define the set of raw device mappings automati-
cally created during the system startup sequence. The format of the file is the same used
in the command line with the exception that the "raw" command itself is omitted.
-q Set query mode. raw will query an existing binding instead of setting a new one.
-a With -q , specifies that all bound raw devices should be queried.
-h provides a usage summary.
The Linux dd (1) command does not currently align its buffers correctly, and so cannot be
used on raw devices.
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 delib-
erate, but is regarded either a bug or a feature depending on who you ask!
Stephen Tweedie (email@example.com)
Version 0.1 Aug 1999 RAW(8)