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RedHat 9 (Linux i386) - man page for raw (redhat section 8)

RAW(8)				     System Manager's Manual				   RAW(8)

       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 -qa

       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 (sct@redhat.com)

Version 0.1				     Aug 1999					   RAW(8)

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