Thursday, March 15, 2018

Volume group types in AIX

Types of Volume group and their characteristic in AIX
Nomal VG- Max PV 32
Big VG- Max PV 128
 Scalable VG- Max PV 1024

Below table give exact Idea types of volume group in AIX.

Maximum PVs
Maximum PPs per VG
Normal VG
32,512 (1016 * 32)
Big VG
130,048 (1016 * 128)
Scalable VG

Convert AIX Volume group from Normal to big using following command

prerequisite before conversion from MAN page of AIX chvg:

The -B flag cannot be used if there are any stale physical partitions.

Once the volume group is converted, it cannot be imported into AIX 4.3.1 or lower versions.

The -B flag cannot be used if the volume group is varied on in concurrent mode.

There must be enough free partitions available on each physical volume for the VGDA expansion for this operation to be successful.
Because the VGDA resides on the edge of the disk and it requires contiguous space for expansion, the free partitions are required on the edge of
the disk. 

#chvg -B vgname


# chvg -B myvg
0516-1164 chvg: Volume group myvg changed.  With given characteristics myvg
        can include up to 128 physical volumes with 1016 physical partitions each.


Convert AIX Volume group from big to scalable using following command

Varyoff vg first then execute command chvg like below.

Prerequisites before converting AIX volume group from Big to scalable
1.No stale partition in VG
2.AIX volume group must be varied off state
3.on AIX VG there must be some free PP.

# varyoffvg myvg

# chvg -G myvg

then varyon vg using below command

# varyonvg myvg

check Volume group type in AIX by using following command

lsvg myvg |grep -i MAX PV

 From MAN page of chvg AIX:

**Once the volume group is converted, it cannot be imported into AIX 5.2 or lower versions.


understanding T-Factor in AIX

AIX admin every time add new disk to volume while adding physical volume to volume group need to concider following calculation.

Total number of PP= disk size / PP size.

max PP per PV in AIX like below

MAX PPs per PV:     1016   //maximum PP in VG where we adding new disk.

suppose PV in volume group rootvg having size 20 GB then above calculation looks like below

Total number of PP on PV =20480 /128

Total number of PP on PV=160

but there is situation while adding PV to volume group you get following error:

**Unable to extend volume group

**use chvg command with -t option to attempt to change the 
maximum Physical Partitions per Physical volume for this volume group

so in above situation T factor comes in picture .

so how to decide T factor:

suppose i want add 200 GB disk in volume group then number of PP on new disk is 1600.

200*1024 (convert to MB)=204800
204800 / 128 (PP size in Vg)     =   1600 PP on new PV.

here this PV will be rejected by VG giving reason that limitation of max PP which is 1016.

to increase PP per PV in volume group command is like below

#chvg -t vgname

factor value = number off PPs on new disk / 1016

factor value= 1600 / 1016

factor value=1.5  //

factor value should be round up so it will be 2.

chvg -t 2 rootvg

after this command  "MAX PPs per PV:  2032"

now after changing T factor AIX admin tried to add the command  will definitely succeed.

AIX chvg T -Factor table for reference :

In short we can say that T-factor increase Number of PP per PV in AIX ,but deceases number of PV in AIX Volume group.

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