Max Physical Volumes 1

[d3funct]

I have conflicting documents pertaining to the -d flag to mkvg. One says that the default is 32 and the maximum is 255 physical volumes, while the other says using the -B flag provides the maximum physical volumes = 128 pv's. Which is correct. Is the Max. possible PV's 255 or 128? Meekness: Uncommon patience in planning a revenge that is worth while.
-- Ambrose Bierce

 

[aixqueen]

I hope this doesn't confuse anyone. The answer is it depends...
in general the rules are below...but they can change for other factors like pp/pv
limits and the 1016 rule...........

-B Creates a big vg format volume group. This can accommodate up to 128
physical volumes and 512 logical volumes.

-d MaximumPhysicalVolumes
Estimates the maximum number of physical volumes contained in the volume group.
The default is 32; a smaller number reduces physical volume space overhead.

Default VGDA size

When the user creates a volume group, the mkvg command defaults to allowing a
maximum of 32 disks in the new volume group. However, as bigger disks
have become more prevalent, this 32-disk limit is usually not achieved
because the space in the VGDA is used up faster, as it accounts for the capacity
on the bigger disks. This maximum VGDA space is a fixed size which is
part of the LVM design. Large disks require more management
mapping space in the VGDA, which causes the number and size of
available disks to be added to the existing volume group to shrink.

When a disk is added to a volume group, not only does the new disk
get a copy of the updated VGDA, but all existing
drives in the volume group must be able to accept the new,
updated VGDA. ]

Another Factor
The disadvantage is that increasing the number of PPs allowed per disk
will limit the number of disks allowed in the volume group. If the 1016 PP/PV
limit is maintained, you can add up to 32 disks per volume group in AIX 4.3.1
and 128 disks in AIX 4.3.2. There is an inherent limit of 32512 PPs in the entire volume group in all levels of AIX EXCEPT version 4.3.2, where the limit is
130048 PPs.

For example: At AIX 4.3.1, if the PP/PV limit is changed to 4064 for a
particular volume group, then only 8 drives
may be placed in the volume group, even if those drives do not
use 4064 PPs each. At AIX 4.3.2, if the Volume
Group was created in a big Volume Group format, and the PP/PV
limit is changed to 4064 for a particular volume
group, then 32 disks may be placed in the Volume Group,
since the maximum number of disks that can be a part of
that Volume Group is now 128.

These docs may help on the VGDA

http://techsupport.services.ibm.com/server/aix.techTips?refNo=90605219814830

http://techsupport.services.ibm.com/server/aix.techTips?refNo=90605226614702

You might want to check out the chvg command as well....
I guess the answer is a general one that there can be other factors
that effect how many disks can be in a volume group.

 

[d3funct]

Thanks for the explanation and links. It was very helpful, though still a little confusing. But I do understand the theory now.

Thanks Meekness: Uncommon patience in planning a revenge that is worth while.
-- Ambrose Bierce