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comp.unix.aix Frequently Asked Questions (Part 3 of 5)
Section - 1.709: How do I remove a volume group with no disks?

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Top Document: comp.unix.aix Frequently Asked Questions (Part 3 of 5)
Previous Document: 1.708: How do I fix Volume Group Locked?
Next Document: 1.710: What are the theoritical limits within the LVM?
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This is a very common question about AIX LVM and I thought
I might take some time to explain what is going on.  Within
a volume group is the Volume Group Descriptor Area (VGDA) is
is kinda a "suitcase" of lvm information.  This is what allows
you to pick up your drives and take them to another machine,
importvg them, and get filesystems automatically defined.

What happens is that when you importvg the volume group,
the RS/6000 goes out and reads the VGDA and finds out about
all the logical volumes and filesystems that may exist on the
volume group.  It then checks for clashes (name conflicts, etc..)
on its own machine and then, here is the important part, populates
its own database with information about the new volume group and
its associated logical volumes.  In cases of filesystems, it will
go into the /etc/filesystems file and add the new filesystem entries
that came along with the imported volume group.

Okay, the key point is that you've got this independent volume group
that has "docked" at the new RS/6000.  What keeps the two tethered
to each other is the varyonvg command.  When this is started on the
volume group, a software link is created where you can't separate the
volume group from the AIX operating system unless the volume group
is no longer seen as active by the system.  In very rare cases, a
situation can occur where the VGDA thinks that someone has it (the
volume group) activated, but the operating system doesn't think it has the
volume group opened up.  This is pretty rare.

The main question I see is "I've taken away the disks, but how do
I get rid of the volume group".  The question should really say,
"How do I get rid of the volume group INFORMATION" since that's
all you have on the system.  You've got possible entries in
the /etc/filesystems and definitely entries in the ODM.  Just 
do:
	exportvg <vgname>

It does a reverse importvg, except it doesn't go off and read
the VGDA.  It nukes anything relating to the volume group in
the /etc/filesystems and ODM.  The only time this won't work is
if the system detects that the volume group is varied on.  Then,
it would be like trying to change tires on a moving car, we won't
let you do it!

Some people are concerned that doing an exportvg will somehow damage
the volume group and/or its VGDA. As I said, all it does is affect the
information about the volume group on the RS/6000 box, not on the actual
disk platter itself.  Thus, the volume group you exported is safe to
take to another system.  The only time the VGDA gets overwritten is when
you create a new volume on top of it.

The second most often asked question is "How do I get rid of a disk
that is no longer really in the volume group?"

In this case, you DON'T want to do an exportvg.  What you want to do
is tell the system you want to cut out the memory of the old, bad disk
from the RS/6000 AND from the VGDA of the volume group.  You simply
do:

		reducevg -d -f <vgname> <hdname>

or if the hdname can't be found:

		reducevg -d -f <vgname> <PVID>

Be careful with this command.  Unlike the exportvg command, actions done
with this command WILL affect the VGDA information on the platter.

Hope this clarifies some questions about volume groups.

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Top Document: comp.unix.aix Frequently Asked Questions (Part 3 of 5)
Previous Document: 1.708: How do I fix Volume Group Locked?
Next Document: 1.710: What are the theoritical limits within the LVM?

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Last Update March 27 2014 @ 02:11 PM