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AIX Volume Group limitations and types (Small, Big, Scalable and how to convert between them)

How To


Summary

This Technote is about limitations of each AIX Volume Group types which are Small (also known as Regular, Original, Normal or Standard VG), Big and Scalable and the requirements to convert from type to another.

What this document Covers:-

Steps

In Logical Volume Management there are 4 main Limits that can limit your volume group which are the following.
  • Maximum number of physical volumes per volume group, Max PVs
  • Maximum number of Logical volumes per volume group,   Max LVs
  • Maximum number of Physical partition per physical volumes , Max PPs per PV
  • Maximum size of Physical partition, Max PP Size
For the Small :-
Max PP Size :- 1 GB
Max LVs :- 256
Max PVs :- 32
Max PPs per VG :- it is the Max PPs per PV x Max PVs
Max PPs per VG :- 1016 x 32 = 32512
For the Big VG :-
Max PP Size :- 1 GB
Max LVs :- 512
Max PVs :- 128
Max PPs per VG :- 1016 x 128 = 130048
For the Scalable VG :-
Max PP Size :- 128 GB
Max LVs :- 4096
Max PVs :- 1024
Max PPs per VG :- 2097152

For a Small or a Big VG we can extend the volume group to have more than 1016 PPs Per PV but this will reduce the number of MAX PVs, these 2 attributes are variable based on the volume group factor also known as "t-Factor". 
The factor is a number between 1-16 for original volume groups, and between 1-64 for big volume groups (factor does not apply to Scalable VG's), For more information about the factor you can check the tech note in the link below :-
If you are unaware of your current VG type you can use the command lsvg “VGname” and notice the MAX LVs and MAX PVs like in the following example :-

# lsvg Test_VG

VOLUME GROUP:       Test_VG                  VG IDENTIFIER:  00f84be500004c0000000175bf034cd9
VG STATE:           active                   PP SIZE:        64 megabyte(s)
VG PERMISSION:      read/write               TOTAL PPs:      575 (36800 megabytes)
MAX LVs:            256                      FREE PPs:       575 (36800 megabytes)
LVs:                0                        USED PPs:       0 (0 megabytes)
OPEN LVs:           0                        QUORUM:         2 (Enabled)
TOTAL PVs:          1                        VG DESCRIPTORS: 2
STALE PVs:          0                        STALE PPs:      0
ACTIVE PVs:         1                        AUTO ON:        yes
MAX PPs per VG:     32512
MAX PPs per PV:     1016                     MAX PVs:        32
LTG size (Dynamic): 256 kilobyte(s)          AUTO SYNC:      no
HOT SPARE:          no                       BB POLICY:      relocatable
PV RESTRICTION:     none                     INFINITE RETRY: no
DISK BLOCK SIZE:    512                      CRITICAL VG:    no
FS SYNC OPTION:     no                       CRITICAL PVs:   no
The volume group in the previous example is a Small VG If you reached the MAX PV limit of a Small VG and the factor already allows the maximum number of PV's for that VG .
You can convert it to Big VG or Scalable VG, you can convert from Small to Scalable directly, but once you converted the volume group to any other type, you cant roll back.
  • A Big VG can accommodate up to 128 physical volumes and 512 logical volumes by default.
  • Changing the VG type to a big VG is an online activity, so you don’t have to vary off the VG.
  • You cannot change the VG type if there are any stale physical partitions or if the volume group is varied on in concurrent mode
  • Changing the volume group to Scalable VG format. This can accommodate up to 1024 physical volumes and 4096 logical volumes.
  • Unlike converting to Big, changing the VG to scalable is an offline activity you will have to vary off the volume group .
  • Once the volume group is converted to scalable it cannot be imported into AIX 5.2 or lower versions.
  • Changing an existing volume group to Scalable VG format will change the device subtype (reported by the IOCINFO ioctl() call) for all associated LVs to DS_LVZ, regardless of the previous subtype. This alteration does not change any behavior of the LV's beyond the reported subtype.
  • In order to change the VG type we need to ensure that there are atleast 1 free PP per PV for large and 2 free PP's per PV for scalable (Recommended to be 2 free PP's per PV for either) because during changing the VG type the VGDA on the disk expands which requires an additional PP atleast.
  • If a logical volume within the VG is striped, the operation might fail to migrate partitions and expand the VGDA.
To confirm if you have enough free PP's per disk use the following command.
# lsvg -p Test_VG
PV_NAME           PV STATE          TOTAL PPs   FREE PPs    FREE DISTRIBUTION
hdisk2            active            511         2           02..00..00..00..00
hdisk3            active            511         21          00..00..00..00..21
hdisk4            active            1023        2           02..00..00..00..00
Once confirmed that you have free PP's on all the disks within the VG then we can proceed with changing the VG type.
# chvg -B Test_VG
0516-1164 chvg: Volume group Test_VG changed.  With given characteristics Test_VG
        can include up to 128 physical volumes with 1016 physical partitions each.

 
Notice the difference in MAX LVs and MAX PVS :-

# lsvg Test_VG

VOLUME GROUP:       Test_VG                  VG IDENTIFIER:  00f84be500004c0000000175bf034cd9
VG STATE:           active                   PP SIZE:        64 megabyte(s)
VG PERMISSION:      read/write               TOTAL PPs:      574 (36736 megabytes)
MAX LVs:            512                      FREE PPs:       574 (36736 megabytes)
LVs:                0                        USED PPs:       0 (0 megabytes)
OPEN LVs:           0                        QUORUM:         2 (Enabled)
TOTAL PVs:          1                        VG DESCRIPTORS: 2
STALE PVs:          0                        STALE PPs:      0
ACTIVE PVs:         1                        AUTO ON:        yes
MAX PPs per VG:     130048
MAX PPs per PV:     1016                     MAX PVs:        128
LTG size (Dynamic): 256 kilobyte(s)          AUTO SYNC:      no
HOT SPARE:          no                       BB POLICY:      relocatable
PV RESTRICTION:     none                     INFINITE RETRY: no
DISK BLOCK SIZE:    512                      CRITICAL VG:    no
FS SYNC OPTION:     no                       CRITICAL PVs:   no
# chvg -G Test_VG
0516-1707 chvg: The volume group must be varied off during conversion to scalable volume group format.
0516-732 chvg: Unable to change volume group Test_VG.
# varyoffvg Test_VG
# chvg -G Test_VG
0516-1224 chvg: WARNING, once this operation is completed, volume group bbvg
        cannot be imported into AIX 5.2 or lower versions. Continue (y/n) ?
...
0516-1712 chvg: Volume group bbvg changed.  bbvg can include up to 1024 physical volumes with 2097152 total physical partitions in the volume group.
If you don’t have enough free PPs on each disk, then you can shrink a filesystem(s) to free some PPs on the VG, and then migrate PPs from the disk(s) that doesn’t have free PPs to other disk(s) within the VG. For more details about this process kindly check the following technote :-

Document Location

Worldwide

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Document Information

Modified date:
03 March 2021

UID

ibm16372968

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