disk(7) 						 Miscellaneous Information Manual						   disk(7)

NAME

       disk - direct disk access

DESCRIPTION

       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 conventions (see intro(7)):

	      Block-mode Devices

	      Character-mode Devices

	      Legacy block-mode Devices

	      Legacy character-mode Devices

       Legacy device special filenames are those used on HP-UX 11i Version 2 and earlier releases.  They can still be used for	backward  compati-
       bility, but only for part of the configuration within the limits of HP-UX 11i Version 2.

       The component parts of the device filename are constructed as follows:

	      N    Required. A decimal number corresponding to the instance number assigned to the direct access device by the operating system.

	      X    Required if is specified. A decimal number corresponding to a partition number.

	      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".
		   Required.

	      Identifies the following hexadecimal digits as a "drive number"
		   or "target".
		   Required.

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

	      Identifies the following hexadecimal digits as a "unit number".
		   Required.

	      n    Hexadecimal unit number within the device.
		   Required.

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

	      m    Required if 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.

   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  sub-
       stantially later in time than their corresponding logical write requests.  This also means that physical read operations may occur substan-
       tially earlier in time than their corresponding logical read requests.

       Block-special files can be read and written without regard to physical disk records.  Block-special file and calls  requiring  disk  access
       result  in one or more byte (typically 2048 byte) transfers between the disk and the block buffer cache.  Applications using the block-spe-
       cial device should ensure that they do not read or write past the end of last 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 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  and  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 and ioctl calls, which are described below.

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

       The and ioctl can be used to obtain device specific identification information.	The information returned includes the disk's model identi-
       fication, the disk interface type, maximum offset address, device type, and the disk's logical block size.

       The ioctl can be used to obtain the capacity of a disk device in units.	is defined in

       The 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 requester).

       WARNING: The ioctl does NOT prevent the use of pass-thru (see scsi_ctl(7)) device files, after the ioctl is issued.

ERRORS

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

	      Required permission is denied for the the device or operation.

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

	      From an	     call: the device is not a disk device.  For other calls: Invalid request or parameter.  Note that for legacy,  32-bit
			     access, this error can result when the size of the device overflows the argument of the or ioctls.

	      If resulting from an
			     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 accessed.

WARNINGS

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

       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 calls to determine appropriate I/O opera-
       tion sizes and alignments.

       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.

AUTHOR

       was developed by HP and AT&T.

SEE ALSO

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

       System Administrator manuals included with your system.

																	   disk(7)