Simple AIX LVM concept and Disk structure

The LVM controls everything here through logical volume device drive. It takes a complex structure of PP, including mirroring, and presents a simple LP to the application.

Physical Volume (hdiskX)

Each physical disk is called a physical volume (PV).

Every PV in use belongs to a VG unless the PV is used as raw storage device or a hot spare.
Each PV contains 1 or more disks (or platters) stacked on top of each other.

PV is also known as direct access storage devices (DASDs). It can be fixed or removable.
A block (sector) is a continuous 512-byte region on PV that corresponds in size to a DASD sector.
A Partition is a set of blocks (with sequential cylinder, head, and sector numbers) contained within a single PV.

Physical Parition

Each PV is divided into PP of a fixed size (e.g. 4Mb partition size).
This is the smallest unit a PV can have.

Volume Group (rootvg, datavg)

Each VG is a collection of 1 or more PV.

A VG can consist of any mixture of physical disk types, though performance is most consistent if devices of the same size are used in a single VG.

PV cannot be shared between VG, therefore entire PV becomes part of the VG.

Each system can have up to 32 VG only.
Information about all LVs and PVs within a VG are stored in VGDA area.
first 64K of a PV is reserved for this area as defined in .
The VGDA consists of bootrecord (first 512 bytes), bad block directory () and LVM record ()

Informations about which PP are stale and which PV are missing within a VG is stored in VGSA area.
The LVM adn SCSI driver reserves somewhere between 7 - 10% of the available disk space for LVM maps, etc.

Lets go deeper.

With each VG, 1 or more LV are defined.

Logical Volume (hd1, hd2var, etc)

Each LV group together 1 or more Logical Partition (LP).

LV is an area of disk used to store data, appearing contiguous to the application but may not be contiguous on the actual PV.
A file system resides on top of an LV, there only 1 LV is mapped to a file system.
LV reside only within VG.
LV can span on multiple PV within the VG.
LV cab also have mirrored copies on different PV within the VG.
Each VG can contain up to 255 LV.

All system LV should reside in the VG named rootvg, under 1 PV if possible. This will allow complete re-installation of the system from backup without affecting the application data that resides on other VG.

There is no difference in response time between a single or multiple VG. When a LV is accessed, the ODM db (/etc/objrepos/*) is searched to determine which VG the LV belongs to, then the VGDA for the VG is read to determine the physical placement of the LV on that PV and ultimately the physical track and sector where the data resides.

Logical Partition

Each LP contains 1 or more physical partition (PP).

If LV is mirrored, then additional PP are allocated.
These mirrors usually reside on different PV (for availability) but may reside on same PV (for performance).

Up to 2 copies of PP can be mirrored, where you have LP count of 3 (2 mirrors plus original)
Space Region in physical Volume

For space allocation purposes, each PV is divided into 5 regions, namely the outer_edge, out_middle, center, inner_middle, and inner_edge. These can be viewed as cylindrical segments cut perpendicularly through the disk platters. The number of physical partitions in each region depends on the total capacity of the disk drive.