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 disk(7)							     disk(7)

      disk - direct disk access

      This entry describes the actions of HP-UX disk drivers when referring
      to a disk as either a block-special or character-special (raw) device.

    Device File Naming Conventions
      Standard disk device files are named according to the following

	   Block-mode Devices		 /dev/dsk/cxtydn[sm]

	   Character-mode Devices	 /dev/rdsk/cxtydn[sm]

      where component parts of the filename are constructed as follows:

	   c	   Required.  Identifies the following hexadecimal digits as
		   the ``Instance'' of the interface card.

	   x	   Hexadecimal number identifying controlling bus interface,
		   also known as the ``Instance'' of this interface card.
		   The instance value is displayed in the ioscan(1M) output,
		   column ``I'' for the H/W Type, ``INTERFACE''.

	   t	   Identifies the following hexadecimal digits as a ``drive
		   number'' or ``target''.

	   y	   Hexadecimal number identifying the drive or target number
		   (bus address).

	   d	   Identifies the following hexadecimal digits as a ``unit

	   n	   Hexadecimal unit number within the device.

	   s	   Optional.  Defaults to that corresponding to whole disk.
		   Identifies the following value as a ``section number''.

	   m	   Required if s is specified.	Defaults to section 0
		   (zero), whole disk.	Drive section number.

      Assignment of controller, drive, logical unit and section numbers is
      described in the system administrator manuals for your system.

 Hewlett-Packard Company	    - 1 -   HP-UX Release 11i: November 2000

 disk(7)							     disk(7)

    Block-special access
      Block-special device files access disks via the system's block buffer
      cache mechanism.	Buffering is done in such a way that concurrent
      access through multiple opens and mounting the same physical device is
      correctly handled to avoid operation sequencing errors.  The block
      buffer cache permits the system to do physical I/O operations when
      convenient.  This means that physical write operations may occur
      substantially later in time than their corresponding logical write
      requests.	 This also means that physical read operations may occur
      substantially earlier in time than their corresponding logical read

      Block-special files can be read and written without regard to physical
      disk records.  Block-special file read() and write() calls requiring
      disk access result in one or more BLKDEV_IOSIZE byte (typically 2048
      byte) transfers between the disk and the block buffer cache.
      Applications using the block-special device should ensure that they do
      not read or write past the end of last BLKDEV_IOSIZE sized block in
      the device file.	Because the interface is buffered, accesses past
      this point behave unpredictably.

    Character-special access
      Character-special device files access disks without buffering and
      support the direct transmission of data between the disk and the
      user's read or write buffer.  Disk access through the character
      special file interface causes all physical I/O operations to be
      completed before control returns from the call.  A single read or
      write operation up to MAXPHYS bytes (typically 64 Kbytes or 256
      Kbytes) results in exactly one disk operation.  Requests larger than
      this are broken up automatically by the operating system.	 Since large
      I/O operations via character-special files avoid block buffer cache
      handling and result in fewer disk operations, they are typically more
      efficient than similar block-special file operations.

      There may be implementation-dependent restrictions on the alignment of
      the user buffer in memory for character special file read() and
      write() calls.  Also, each read and write operation must begin and end
      on a logical block boundary and must be a whole number of logical
      blocks in size.  The logical block size is a hardware-dependent value
      that can be queried with the DIOC_DESCRIBE ioctl call, which is
      described below.

      In addition to reading and writing data, the character-special file
      interface can used to obtain device specific information and to
      perform special operations.  These operations are controlled through
      use of ioctl calls.  Details related to these ioctls are contained in

      The DIOC_DESCRIBE ioctl can be used to obtain device specific
      identification information.  The information returned includes the
      disk's model identification, the disk interface type, and the disk's

 Hewlett-Packard Company	    - 2 -   HP-UX Release 11i: November 2000

 disk(7)							     disk(7)

      logical block size.

      The DIOC_CAPACITY ioctl can be used to obtain the capacity of a disk
      device in DEV_BSIZE units.  (DEV_BSIZE is defined in <sys/param.h>).

      The DIOC_EXCLUSIVE ioctl can be used to obtain and release exclusive
      access to a disk device.	Exclusive access is required for some
      special operations, such as media reformatting, and may be desirable
      in other circumstances.  The value one specifies that exclusive access
      is requested.  The value zero specifies the exclusive access should be
      released.	 Exclusive access causes other open requests to fail.
      Exclusive access can only be granted when the device is not currently
      opened in block-mode and there is only one open file table entry for
      that disk device (the one accessible to the exclusive access

      The following errors can be returned by a disk device driver call:

	   [EACCES]	  Required permission is denied for the the device
			  or operation.

	   [ENXIO]	  If resulting from an open() call, this indicates
			  there is no device at the specified address.	For
			  other calls, this indicates the specified address
			  is out of range or the device can no longer be

	   [EINVAL]	  From an open() call: the device is not a disk
			  device.  For other calls: Invalid request or

	   [EIO]	  I/O error (e.g., media defect or device
			  communication problem).

      The interaction of block-special and character-special file access to
      the same BLKDEV_IOSIZE-sized block is not specified, and in general is

      On some systems, having both a mounted file system and a block special
      file open on the same device can cause unpredictable results; this
      should be avoided if possible.  This is because it may be possible for
      some files to have private buffers in some systems.

      Although disk devices have historically had small (typically 512-byte)
      block sizes, some disk devices (such as optical disks and disk arrays)
      have relatively large block sizes.  Applications using direct raw disk
      access should use ioctl() calls to determine appropriate I/O operation
      sizes and alignments.

 Hewlett-Packard Company	    - 3 -   HP-UX Release 11i: November 2000

 disk(7)							     disk(7)

      Any disk with removable media (for example, floppy or CD-ROM)
      containing a mounted file system should not be removed prior to being
      unmounted.  Removal of disk media containing mounted file systems is
      likely to result in file system errors and system panics.

	   Devices whose logical block size is less than DEV_BSIZE must be
	   accessed on DEV_BSIZE boundaries and with transfer sizes that are
	   multiples of DEV_BSIZE.  Disk ``sections'' 0 (zero) and 2 (two)
	   have exchanged meanings with HP-UX Release 10.0 and beyond.
	   Whole disk is section 0.

      disk was developed by HP and AT&T.

      mknod(1M), intro(7), ioscan(1M).

      System Administrator manuals included with your system.

 Hewlett-Packard Company	    - 4 -   HP-UX Release 11i: November 2000