Assigns rawdevices to blocks for use.
Now lets see the manual of rawdevices service.Manual rawdevices:NAME
raw - bind a Linux raw character deviceSYNOPSIS
raw /dev/raw/raw<N> <major> <minor>
raw /dev/raw/raw<N> /dev/<blockdev>
raw -q /dev/raw/raw<N>
Although Linux includes support for rawio, it is now a deprecated interface. If your application performs device
access using this interface, Red Hat encourages you to modify your application to open the block device with the
O_DIRECT flag. The rawio interface will exist for the life of Red Hat Enterprise Linux 4, but is a candidate for
removal from future releases.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
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, 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.
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.
Use the /etc/sysconfig/rawdevices file to define the set of raw device mappings automatically 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.OPTIONS
-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.BUGS
The Linux dd (1) command should be used without 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
Stephen Tweedie (firstname.lastname@example.org)