Q 1. While creating Volume Group can I use Physical Volumes which are part of other Volume Groups?
Answer: No and Yes.
No: The Physical Volumes specified are checked to see if these are part of other Volume Groups. If they are used in other Volume Groups, the command terminates without creating Volume Group.
Yes: If you specify -f option while creating a new Volume Group, then its possible. See what happens.
Let’s say you have issued the following command:
mkvg -f -y my_vg hdisk2 hdisk3 hdisk4

Let’s say hdisk3 is part of another Volume Group: my_other_vg. Here two things happen:

1. If the Volume Group: my_other_vg is varied off at the time of creating new Volume Group my_vg, the Volume Group my_vg will be created with the Physical Volumes hdisk2, hdisk3 and hdisk4. But, the previous contents of hdisk3 will be lost.
2. If the Volume Group: my_other_vg is varied on, the command will terminate and Volume Group my_vg will not be created.

Q2. Which users can create a Volume Group?
Answer: The user should be either root or someone belonging to System group.

Q3. Is it necessary to specify the Physical Partition size while creating Volume Group?
Answer: Obviously No. The mkvg command automatically adjusts the size of Physical Partitions according to the size of Physical Volume. If however you need to specify the size of Physical Partitions, then keep in mind that it should be large enough to accommodate the full Physical Volume.

For example: if suppose you have physical volume of size 130GB the Physical Partitions size is automatically adjusted to 128MB by the system. If you need to change it to 64MB, then specify -s 64 and -t 2. See this article: How To Create Volume Group to know more.

Q4. How can I specify the Physical Partitions size less than the default size?
Answer: Refer to Q3.

Q5. While creating Volume Group can I use Physical Volumes which are part of third party Volume Managers?
Answer: No: The command terminates if it detects that the Physical Volume specified is being used by some other third party Volume Manager e.g VxVM(Veritas Volume Manager).

Q6. How can I use the Physical Volume in Volume Group creation which was in use of some other third party Volume Manager?
Answer: You have to clear the Physical Volume of the third party volume manager. Specify -C option with chpv as below:
chpv -C hdisk3

Q7. Does Volume Group varyon always automatically?
Answer: No. By default the Volume Group varies on automatically after creation.
In following two conditions, the Volume Group will not varyon automatically:

1. If you specify -n option with mkvg command.
2. If you specify -C option, to make it concurrent capable, then it’s not varied on automatically

Q8. Can I specify total number of Physical Partitions in Volume Group at the time of creation?
Answer: Yes. You can use -P option. The valid values are: 32, 64, 128, 256, 512, 1024, 2048. Specifying any value will limit maximum number of physical partitions to: valueX1024 partitions. e.g 32 will limit maximum number of physical partitions in Volume Group to : 32X1024=32768 partitions.

Q9. Can I specify total number of Logical Volumes in Volume Group at the time of creation?
Answer: Yes. You can use -v option. The valid values are: 256, 512, 1024, 2048. Specifying any value will limit maximum number of Logical Volumes in Volume Group to : value Logical Volumes. e.g 512 will limit maximum number of Logical Volumes in Volume Group to : 512 Logical Volumes.

Q10. Can I import Volume Group created in AIX 5.3 to AIX 5.2 or lower?
Answer: By default No. But if you want to do it, specifying -I option at the time of creation of Volume Group, you will be able to import in AIX 5.2 or lower.

Q11. Are there any conditions when I can not import Volume Groups created in AIX 5.3 to lower versions of AIX?
Answer: Yes. In the following two conditions:

1. If Volume Group is Big VG, created with -B flag then you can not import it to AIX 4.3.1 or lower.
2. If Volume Group is Scalable VG, created with -S flag then you can not import it to AIX 5.2 or lower.

To add a Volume Group, we need to have few things in mind:

1. The name of Volume Group.
2. Whether you want to limit the maximum number of Physical Volumes in this Volume Group.
3. Names of Physical Volumes used while creating Volume Group.
4. The size of Physical Partition.

I assume the size of all Physical Volumes to be hdisk2 and hdisk3 each of size 130GB.

How To Create A Volume Group
If you want to keep default settings the Physical Partition size will be: 130*1000/1016 MB which turns about to be around 128MB.

So, you VG creation command will be:
mkvg -y my_vg hdisk2 hdisk3

How To Create A Volume Group With More Than 1016 Physical Partitions Per Physical Volume
Now, let’s say you want to have maximum number of Physical Partitions to be more than 1016 or you want lower than 128MB Physical Partition size, you can use t-factor. The t-factor helps you increase the maximum number of Physical Partitions per Physical Volume. But it decreases the maximum possible Physical Volume in a Volume Group. If you could have 32 Pvs previously, with a t-factor of 2, you will have now maximum of 16 Pvs. T-factor can only be an integer.

Now see what happens:

Maximum no. of PPs: 2*1016=2032
Size of PV: 130GB.
Size of single PP:130*1000/2032=64MB

So, you will need to specify s also, which stands for the size of Physical Partition to use. Which in our case will be 64MB.

mkvg -s 64 -t 2 -y my_vg hdisk2 hdisk3

How To Create A Big Volume Group
To create a Big Volume Group, specify -B option. With this option you can have maximum of 128 Physical Volumes and 512 Logical Volumes.
Issue the command:
mkvg -B -v 512 -y my_vg hdisk2 hdisk3

-v option is used to specify the number of Logical Volumes. The valid values of -v are: 256, 512, 1024 and 2048.

How To Create A Scalable Volume Group
To create a Scalable Volume Group, specify -S option. With this option you can have maximum of 1016 Physical Volumes and 2048 Logical Volumes.
Issue the command:
mkvg -S -v 2048 -y my_vg hdisk2 hdisk3

How To Create A Volume Group With A Given Major Number
Let’s say for example you want to create a Volume Group with a Major Number 43.
Issue the command:
mkvg -V 43 -y my_vg hdisk2 hdisk3

To know the major number, you should issue the command:
wiw_labs:$ lvlstmajor
43…

How To Create A Volume Group And Accept Physical Volumes From Standard Input
To accept list of Physical Volume from standard input you need to issue mkvg command with -i option.
echo “hdisk2 hdisk3″ | mkvg -i -y my_vg

How To Create An Enhanced Concurrent Capable Volume Group

For enhanced concurrent capable Volume Group you need to issue mkvg command with -c option. See this article for more information on types of Volume Group. .

mkvg -c -y my_vg hdisk2 hdisk3

If you are using HACMP then use:
mkvg -C -y my_vg hdisk2 hdisk3

How To Create A Volume Group Which Does Not Vary on Automatically
For creating a Volume Group which does not varyon automatically, you need to issue mkvg command with -n option.
mkvg -n -y my_vg hdisk2 hdisk3

Here are few things which I’m gonna discuss in this post:

How To List Volume Groups
Using command lsvg provides the complete list of Volume Groups available in the system. That does not matter whether the volume group is varied on or varied off.
e.g.
wiw_labs:$ lsvg
altinst_rootvg
rootvg
myvg
appvg

How To List Only Varied On Volume Groups
Using command lsvg, with -o option provides the list of Volume Groups which are currently varied on. Varied on means the Volume Group is open for use.
e.g.
wiw_labs:$ lsvg -o
rootvg
appvg

How To List Volume Group Characteristics
To list characteristics of a particular Volume Group, we use the lsvg command with -l option and name of Volume Group.

e.g wiw_labs:$ lsvg appvg

Below is explained the meaning of various terms used in the listing:

1. Volume Group: Name of Volume Group.
2. VG State: active/partial. Partial indicates that some of the Physical Volumes in the Volume Group are not active. See this for more on VG state.
3. VG Permission: It indicates if the volume group is read-only or read-write.
4. Max LVs: Maximum number of Logical Volumes allowed in Volume Group.
5. LVs: Total number of Logical Volumes at present in Volume Group.
6. Open LVs: Number of LVs open at present in the Volume Group for any Read Write operation.
7. Total PVs: Total number of Physical Volumes present in Volume Group.
8. Stale PVs: How many stale Physical Volumes are there in Volume Group. The stale Physical Volumes are those ones on which I/O operations are not possible.
9. Active PVs: Total number of Physical Volumes which are active in Volume Group.
10. Max PPs per VG: Maximum no. of Physical Partitions which can be there in the Volume Group.
11. Max PPs per PV: Maximum number of Physical Partitions which can be there in Physical Volume.
12. VG Identifier: The very long string to identify Volume Group.
13. PP Size: The size of Physical Partition used in every Physical Volume of Volume Group.
14. Total PPs: Total number of Physical Partitions in Volume Group.
15. Free PPs: Total number of free Physical Partitions with in Volume Group.
16. Used PPs: Total number of Physical Partitions in use in Volume Group.
17. Quorum: Whether the quorum is on or not in Volume Group.
18. VG Descriptors: Number of Volume Group Descriptor Areas(VGDA) in Volume Group.
19. Stale PPs: How many Physical Partitions are stale or unable to perform I/O operations.
20. Auto On: Whether the Volume Group will varyon on system startup or not.
21. Max PVs: Maximum number of Physical Volumes allowed in this Volume Group.
22. Auto Sync: Whether this Volume Group should sync automatically or not.
23. BB Policy: Its bad block relocation policy. So, this value here is relocatable, that means we are allowed to relocate bad blocks in this Volume Group if need be.

(Refer AIX Manpages for more details.)

Why Does Logical Volume Become Fragmented
A file system needs to be created on a Logical Volume to make it usable. When we run out of space on a File System, we have to add additional space to the Logical Volume, with which the file system is related. Now, Logical Volume’s space is increased by adding new Logical Partitions to it, which in turn point to Physical Partitions on Physical Volumes. Its not necessary that the Logical Partitions, which we are providing to a Logical Volume are close to its previous Logical Partitions. The very next Physical Partitions available on Physical Volume are assigned to Logical Partitions and those Logical Partitions are assigned to Logical Volume. As a result fragmentation occurs in a Logical Volume with time. This leads to performance issues later on.

What is Volume Group Reorganization
To get best performance from a Volume Group, its required that all the Logical Partitions within respective Logical Volumes must be contiguous. Or to say it more clearly, the Physical Partitions should be contiguous. For this purpose we need to reorganize the Volume Group. In this process, the Physical Partitions within Physical Volumes are assigned to Logical Volumes as per their characteristics. The net result is that the Logical Partitions point to Contiguous Physical Partitions within the Logical Volume.

Command Used To Reorganize A Volume Group
The command provided by Aix is reorgvg. The reorgvg command is used to reorganize the VG. The general format is:

reorgvg [-i] VG [LV1 LV2 LV3...]

-i: This option specifies that the input Physical Volumes comes from standard input. Only LVs within those Physical Volumes will be reorganized.
e.g
reorgvg my_vg lv2 lv5 lv9

This command is to reorganize Logical Volumes lv2, lv5 and lv9 within Volume Group my_vg.

If you want to specify that Physical Partitions from particular Physical Volumes only should be reorganized then you give command:

echo “hdisk2 hdisk3″ | reorgvg my_vg lv2 lv5 lv9

Now, whichever Physical Partitions related to lv2, lv5 and lv9 reside on hdisk2 and hdisk3, will be reorganized.

Things To Take Care About reorgvg Command
There are several things which should be taken care of while using reorgvg command:

1. At least one Physical Partition should be free on Physical Volume. Otherwise command will fail.
2. User should be either root user or system group member.
3. if no arguments are given to reorgvg command, the whole Volume Group is reorganized.

4. The reorganization flag of Logical Volume Should be set to Yes. Use:

   chlv –r lv   to do this, it’s not enabled.

What is The Limitation of reorgvg Command
The command has limitation that it does not recognize the placement of allocated Physical Partitions for striped Logical Volumes. As a result you may not run command on striped metaluns.

Is This reorgvg Command Disallowed Somewhere
Yes, The command is not allowed on snapshot Volume Group. Also the Volume Group should not be concurrent.

How To List Physical Volumes In Aix
To list Physical Volumes in Aix lspv command is used. This command gives the output as name of Physical Volume, its identifier which is a 16 character serial number, the Volume Group to which it belongs and the state of the physical volume whether its active or not. The identifier is also called PVID. Below is the command:

wiw_labs:$ lspv

hdisk0 034cd34d03b25531 altinst_rootvg

hdisk1 034cd34d7533e52b altinst_rootvg

hdisk2 034cd34d753ca221 rootvg active

hdisk3 034cd34d753ca5a8 rootvg active

How To List Connectivity Information of Physical Volume
To see the connectivity information of a Physical Volume you need to issue lsdev command. Here is how you issue this command:

wiw_labs:$ lsdev -C -c disk
hdisk0 Available 23-06-00-6,0 16 Bit LVD SCSI Disk Drive
hdisk1 Available 3p-06-00-6,0 16 Bit LVD SCSI Disk Drive
hdisk2 Available 4b-06-00-6,0 16 Bit LVD SCSI Disk Drive
hdisk3 Available 5M-06-00-6,0 16 Bit LVD SCSI Disk Drive

-C: Device Class
-c: Device subclass(disk in this case)

Here you see the name of the disk, the status of the disk(available), its connectivity information and type of the disk.

How To List Overall Information About A Physical Volume
To see the overview of a physical volume let’s say hdisk2, we issue the command lspv with hdisk2 as argument as below:

wiw_labs:$ lspv hdisk2

*(Verify the distribution parameters by adding the PPs in a particular region and then check in overall information.)

The above command gives information about name, state, ID, Physical Partition Size, No of PP, Free and used PP, distribution of PPs, VG, allocatability and other parameters.

How To See What Logical Volumes Are Present On Physical Volume
To see the Logical Volume information on a Physical Volumes, you need to issue the command lspv with -l(logical volume) option. This gives information of Logical Volume name, no. of Logical Partitions in LV, No. of Physical Volume, the allocation distribution and mount point. Here is an example output:

wiw_labs:$ lspv -l hdisk2

How To See Physical Partition Information of Physical Volume
To see the Physical Partition information of a particular Physical Volume, you need to issue lspv command with -p flag.

wiw_labs:$ lspv -p hdisk2

PP Range: Tells about Physical Partitions being used.
State: Whether PP is used or free.
Region: Where the PP is located on disk: outer edge, outer middle, center, inner middle or inner edge.
LV: Which Logical Volume is using these PPs.
Type: What type of Logical volume is there like boot, sysdump, jfs2, jfslog, paging etc.
Mount Point: It’s the mount point where a particular LV is mounted.

How To Determine Which Physical Partition is Mapped To Which Logical Partition
To determine the partition mapping from PP to LP, you need to issue lspv command with -M option. Let’s see the output below:

wiw_labs:$ lspv -M hdisk2

The output is truncated, because of space considerations.

The output shows which Physical Partition on hdisk2 is mapped to which Logical Partition on a Logical Volume.

What Are The Components of A File System
The File System normally has the below mentioned components, depending upon the partition/logical volume and Operating System type:

1. Boot Block.
2. Super Block.
3. Bitmaps.
4. Allocation Groups.

 
Aix Supported File Systems
The Aix OS supports mainly three types of File Systems:

1. Journaled File System(JFS/JFS2).
2. Network File System(NFS).
3. CD-ROM File System(CDRFS).

 
Journaled File System
The Journaled File System is native to AIX. This File System retain a journal for every I/O transaction on a Physical Volumes. One Logical Volume can contain only one Journaled File System. In order to use the File System, the Operating System has to mount it. The File Systems which are critical to the booting and operation of the Operating System, are mounted during boot time operation.

The Journaled File Systems are of two types: JFS and JFS2. AIX 5.3 onwards JFS2 is the default File System. Earlier it used to be JFS. JFS2 has some enhancements over JFS.

Network File System
The NFS is used to allow the users to access the files and directories over the network. NFS has been in use since the very beginning for almost all kind of *nix Operating Systems. Almost every *nix OS supports NFS. NFS is a distributed File System and provides seamless access to the files from the server side.

CD-ROM File System
The CDRFS is the one typical to CDs.

Journaled File System Structure
The entire JFS is nothing but a set of logical blocks.

0Boot Record: This is the first logical block in the File System and is reserved for bootstrap program. File System does not use it.

Superblock: The super block keeps and maintains information about total File System Its size is 4096Bytes. It has several types of information:

1. File System size.
2. Total Number of data blocks in File System.
3. Flag to indicate the state of File System.
4. Allocation Group Sizes.

 
Allocation Groups: The total File System is a collection of logical blocks. After leaving initial few logical blocks, the logical blocks in rest of the File System are grouped together. Then additional space is allocated in group of these logical blocks. That’s why these are called allocation groups. The i-nodes refer to these data blocks. So, allocating the i-nodes means allocating these groups of data blocks.

Logical Volume Defined
Logical Volumes can be defined as the logical entities which are created from Logical Partitions. While creating Logical Volumes, the number of Logical Partitions are specified.

Mapping To Physical Partition
One Logical Partition maps exactly to one Physical Partitions. But if it’s the case of mirroring then one Logical Partition can map to two or three Physical Partitions as well.

Size of Logical Volume
The size of Logical Volume is determined by the following formula:
Size of LV = Size of LP * No. of LVs * No. of copies of LPs

Components of Logical Volume
Each of the Logical Volume has a Logical Volume Identifier(LVID). It is used to recognize the Logical Volume while using LVM libraries and certain low level commands. LVID format is: VGID.num. The num  is the number used to represent the Logical Volume.

Contents of Logical Volume
Each of the Logical Volume contains Logical Volume Control Block. Its better known as LVCB. LVCB takes the first 512 Bytes of Logical Volume. All of the information about Logical Volume is contained there-in. The information is:

1. Time of creation.
2. Information about mirroring.
3. Mount Point(If it has File System created on it.).

 
I’ll discuss the operations possible on Logical Volumes in some other post.

The Volume Groups are totally logical entities. And a number of operations can be carried out on Volume Groups.

Below are the Volume Group characteristics.

Naming of Volume Group: The name of Volume Group should be unique and the name should range from 1 to 15 characters.

States of Volume Group: The following states are possible for Volume Group:

1. Active/Complete.
2. Active/Partial.
3. Inactive.

 
Active/Complete State
The active/complete state of Volume Group signifies that all of the physical volumes in the Volume Group are fully active. In this state the Volume Group is fully usable by the applications. The varyonvg command is used to bring the Volume Group to active/complete state.

Active/Partial State
The active/partial state of Volume Group signifies that all of the physical volumes in the Volume Group are not active. Some of the PVs can be in unavailable or removed state. This may cause the alarming condition and the data loss may happen if continuing in this state.

Inactive State
The inactive state of Volume Group signifies that the Volume Group is not in use and can’t be used for any application. The varyoffvg command used to bring down the Volume Group.

Access Permissions For Volume Group
The Volume Group can be varied on with two types of permissions:

1. Read-only.
2. Read-write.

 
Type of Volume Group
The Volume Groups are of three type:

1. Normal Volume Group.
2. Big Volume Group.
3. Scalable Volume Group.

 
Normal Volume Group
Normal Volume Group can have up to 32 Physical Volumes and 255 Logical Volumes.

Big Volume Group
Big Volume Group can have 128 Physical Volumes and 512 Logical Volumes.

Scalable Volume Group
Scalable Volume Group can have by default 1024 Physical Volumes, 256 Logical Volumes and 32768 Physical Partitions. The number of Logical Volumes can be increased up to 4096and Physical Partitions can also be increased.

Classification on The Basis of Concurrency
I am just listing these. If you want more details please see my previous article Non Concurrent, Concurrent and Enhanced Concurrent Volume Groups. The volume groups can be classified as:

• Non-Concurrent Volume Group.
• Concurrent Volume Group.
• Enhanced Concurrent Volume Group.

Next time I’ll be discussing some other aspects of Volume Groups.

Apart from the third party available solutions for protecting AIX data integrity, AIX has a number of built-in functions to prevent loss or corruption of data on Physical Volumes. The native data protection solutions provided by AIX are:

1. Bad Block Relocation.
2. Mirroring.
3. Quorum Checking.

 
Here in the article we’ll be discussing Bad Block Relocation.

Bad Block Relocation

AIX is intelligent enough to detect bad blocks and then to relocate the data on disk drives. Whenever there is some problem on disks, the data relocation takes place automatically for data protection. The data relocation occurs at three levels:

1. Data relocation within the disk.(Seamless)
2. Data relocation by hardware.(By PV)
3. Software relocation by software.(By Device Driver)

 
Data Relocation Within The Disk
This is least complicated operation for data relocation. If the system fails to carry out data I/O requests, then it anticipates data corruption. So data protection is carried out by moving data automatically within disk. No notifications are generated.

Hard Data Relocation
This is slightly more complex way of data relocation. The operation is initiated and instructed by LVM device driver, but carried out by Physical Volume. In this case data is moved from one Physical Partition to another reserved physical partition. So, data location changes from one PP A to another PP B. After the completion of data relocation, the device driver continues to refer to old location, PP A, but  the Physical Volume handles the I/O operation from the second location, PP B.

Soft Data Relocation
This is the complex most way of data relocation. The operation is initiated and carried out by LVM device driver. Bad block directory is maintained by LVM  device driver. Also it maintains the translation table which maps the bad block PP A to good block PP B. Let’s say PP A location was bad block. So, it will be in the bad block directory of LVM  device driver. Whenever some request comes for some I/O operation for any block, the device driver will look its bad block directory. If the block is listed in that, it translates the location requested to corresponding good location, say PP B, where the actual data is residing. Hence data protection is ensured in most cases.

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.