HP-UX Reference (11i v2 04/09) - 7 Device (Special) Files, 9 General Information, Index (vol 10)

d
disk(7) 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:
Block-mode Devices
/dev/dsk/cxtyd
n[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’’.
Required.
t Identifies the following hexadecimal digits as a ‘‘drive number’’ or ‘‘target’’.
Required.
y Hexadecimal number identifying the drive or target number (bus address).
Required.
d Identifies the following hexadecimal digits as a ‘‘unit number’’.
Required.
n Hexadecimal unit number within the device.
Required.
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.
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 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
read() and write() calls requiring disk access result in one or more BLKDEV_IOSIZE byte (typi-
cally 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 inter-
face 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 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
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
Section 7−−24 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: September 2004