AIX Logical Volume Manager A Brief Introduction

The storage in Aix can be divided in two layers: Physical and Logical.

On physical level the two things come into picture:

1. Physical Volume(PV).
2. Physical Partition(PP).

On logical level the storage is divided in three parts:

1. Logical Partition(LP).
2. Logical Volume(LV).
3. Volume Group(VG).

I’m going to discuss the terminology here:

Physical Volume
The hard disk in LVM’s language is called Physical Volume. It has unique identifier. There is a limitation as to how many physical volumes can be there in AIX. Various commands can be used for PVs like lspv, chpv etc. While in use, the application is not aware, where the data is stored on hard disk.

Physical Partition
The physical volume is divided in a number of small equal partitions. These are called physical partitions. These are used in LVM for making up the components like LP, LV, VG etc.

Logical Partition
The logical aspect of LVM starts at this level. Each logical partition corresponds to one and only one physical partition. So we can say: LP —-> PP and PP —->  LP. This is the unit which is used to increase or decrease the size of file systems in  OS.

Logical Volume
The logical volume is composed of several Logical Partitions. Thus the size of Logical Volume can always be calculated as below:

Size of LV = Number of LPs  X   Size of LP(Which is equal to PP).

Number of Logical Volumes are restricted by the type of Volume Group.
The logical volume is usable only if the File System is created on it. Otherwise its not usable. The file systems normally used in AIX are jfs and jfs2. These are journaling based file systems. Means, whatever I/O operations are done on filesystem, all are recorded. The record is called journal.

Volume Group
The volume group is the highest entity in LVM hierarchy. VG is composed of several LVs. While creating VGs, the PP size is decided and that can not be changed throughout the life of VG. VGs are of three types:

1. Normal VG.
2. Big VG.
3. Scalable VG.

More about VGs and their details will be written in some future posts. That’s all for now.