disk(7)disk(7)NAMEdisk - 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 conven‐
tions (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 compatibil‐
ity, 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 operat‐
ing 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, col‐
umn "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 buf‐
fer cache permits the system to do physical I/O operations when conve‐
nient. This means that physical write operations may occur substan‐
tially 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 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-special
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 sup‐
port 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 opera‐
tions, 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 iden‐
tification, 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 argu‐
ment of the or ioctls.
If resulting from an
call, this indicates there is no device at the
specified address. For other calls, this indi‐
cates 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 operation sizes
and alignments.
Any disk with removable media (for example, floppy or CD-ROM) contain‐
ing 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 ALSOioscan(1M), mknod(1M), scsi_ctl(7), intro(7).
System Administrator manuals included with your system.
disk(7)
