Physical volumes, also known as direct access storage devices (DASDs), are
fixed or removable storage devices. Typically, these devices are hard disks. A
fixed storage device is any storage device defined during system
configuration to be an integral part of the system DASD. The operating
system detects an error if a fixed storage device is not available at some time
during normal operation. A removable storage device is any storage device
defined during system configuration to be an optional part of the system
DASD. The removable storage device can be removed from the system at any
time during normal operation. As long as the device is logically unmounted
first, the operating system does not detect an error.

The following are terms used when discussing DASD volumes:
Block A contiguous, 512-byte region of a physical volume that
corresponds in size to a DASD sector. This is often used
interchangeably with sector.
Partition A set of blocks (with sequential cylinder, head, and sector
numbers) contained within a single physical volume.

A physical volume is a DASD structured for requests at the physical level, that
is, the level at which a processing unit can request device-independent
operations on a physical sector address basis. A physical volume is
composed of the following:
• A device-dependent reserved area.
• A variable number of physical blocks that serve as DASD descriptors.
• A number of partitions, each containing a fixed number of physical blocks.

Typical operations at the physical level are read-physical-block and
write-physical-block.

The number of blocks in a partition, as well as the number of partitions in a
given physical volume, are fixed when the physical volume is installed in a
volume group. Every physical volume in a volume group has exactly the same
partition size.

A disk must be designated as a physical volume and put in an available state
before it can be assigned to a volume group. A physical volume has certain
configuration and identification data written on it.

Sometime, it can get confusing as to the true name of a physical volume and
how it is recognized by the system. This usually occurs during service calls or
after a system has crashed. The ODM is used to attach an hdiskx name to a
physical volume ID (PVID) of a disk. The PVID is the soft “serial number” of a
disk that is created, usually just once during the lifetime of a physical
volume’s usage on AIX. It may need to be recreated using the dd command if
someone accidently overwrites it, or uses the low level diagnostics to
reformat the disk.

To make a disk into a physical volume, the PVID is placed onto the disk. The
PVID is an combination of the machine’s serial number (from the systems
EPROMs) and the date the PVID was generated. This combination ensures
the extremely low chance of PVIDs being duplicated. When the system is
booted, the disk configurator looks at the PVID residing on the disk and
compares it with an entry in the ODM. If an entry is found, then the disk is
given the hdiskx number in the ODM that is associated with the PVID. If there
is no matching entry, then the next name in the pool of ’free’ hdisk names is
allocated to the physical volume.

When you add a physical volume to a volume group, the physical volume is
partitioned into contiguous, equal sized units of space called physical
partitions. A physical partition is the smallest unit of storage space allocated.
Physical volumes inherit the volume group’s physical partition size, which can
only be set when the volume group is created (for example, using mkvg -S).

The support for physical partition sizes of 512 MB and 1024 MB have been
added to AIX 4.3.1. Hence, if you add a new volume group, the physical
partition sizes in megabytes you are allowed to chose are: 1, 2, 4, 8, 16, 32,
64, 128, 256, 512, and 1024 MBs.

Advertisements