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
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
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 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
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
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
[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
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
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