UNIX for Dummies Questions & Answers

Hi,
I am new to unix. I am working on Red Hat Linux and side by side on AIX also. After reading the concepts of Storage, I am now really confused regarding the terminologies

1)Physical Volume
2)Volume Group
3)Logical Volume
4)Physical Partition

Please help me to understand these concepts.

Hi, kashifsd17.

Welcome to the forum. We prefer that you do some searching to try to become a more independent problem-solver.

So next time, please try Google, etc..

However, one of the best briefings I have found is with many diagrams at:

Logical volume management

Best wishes ... cheers, drl

Hi Drl,
Thanks for the reply. Actually I searched a lot but not got any satisfying answer to my question anywhere.That is why I posted it here.

The most important concept to grasp is that a Logical Volume within a Logical Volume Group can extend across multiple Physical Partitions of multiple Physical Volumes.

Tip: A Physical Volume from the point of view of your computer is a hard disc drive. In this modern age your computer could see a Storage Array (or part of a Storage Array) as one Physical Volume (though the Storage Array is actually composed of multiple discs).
i.e. When considering Physical Volumes always consider your computers view (looking out) regardless of the physical device.

The controlling software is Logical Volume Manager (LVM).

Ps: My first LVM course (albeit from Veritas) took 5 days. It's a big subject.

Thanks Methyl. Please correct me if I am wrong,
A physical volume = hard disk
Volume Group = total storage on a machine
Logical volume = A Partition on volume group
Is it so ?

Volume Group = An administrative group of Logical Volumes. A commercial computer can (and usually does) have multiple Volume Groups.
The sum of the size of alll the Volume Groups would be the total size of all the formatted disc space controlled by LVM.
You don't have to give all your discs up to LVM control, but there is rarely a reason to not give up all your Physical Volumes to LVM control.

Conventionally VG00 (Volume Group zero) is the system Volume Group (e.g. Logical Volumes: root, /usr, /var, /opt, /tmp ... and sometimes /home. No system account such as the root account should have its home directory on /home).

How you group your volumes after that is personal preference. I tend to use VG01 for user home directories and data, and VG02 onwards for databases. This is not a rule, just personal preference.
The Volume Groups become very important when designing backup strategies.

Physical Volume = From the point of view of your computer a physical hard disc.

Logical Volume = A logical disc which may spread across multiple physical discs. Your unix system will mount a Logical Volume on a mountpoint just as if it was a physical disc partition.

Hope this helps.

There are two major concepts in LVM to grasp:

Striping.
Where you deliberately create a Logical Volume across partitions on multiple physical discs. This can give a dramatic performance improvement.

Mirroring.
Where you have one (or preferably more) mirror copies of your critical Logical Volumes on totally different physical disc drives.
Disc drives do fail. With careful design for resilience you can keep running with a failed disc drive and replace a hot-pull disc without interruption to service. Taking this concept further you can fit multiple hot spare disc drives and configure the LVM system to automatically replace the failed disc drive.

Given the option of performance against resilience I would choose resilience every time.

Don't forget to check your server for failed disc drives at least once a day.

Ps. Checking resilent Disc Arrays is equally important because a failure will not be visible to your unix system.