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hello,

I am looking at an aix 5.1 system in which the prior administrator mirrored
the rootvg over two physical volumes such that each logical volume in the
rootvg had a single copy of itself on a different PV. So essentially the
whole rootvg appears to be simply replicated on another drive. It would
seem however, that the quorum on the rootvg appears to be negating the
entire effort of having such a mirror for the quorum is listed as (lsvg
rootvg)

QUORUM: 2

now I may have this wrong since its been a while since I have administrated
aix volume groups, but my notes and my memory tell me that this is the
number of disks required to keep the volume group accessible? even if that
is incorrect it still seems that quorum checking is enabled on the rootvg.
if i look at it in smit i see

A QUORUM of disks required to keep the volume yes
+
group on-line ?

this is why im guessing that the "QUORUM: 2" in lsvg rootvg pertains
to the number of disks in the quorum (am I right on this interpretation of
that line in lsvg rootvg?)


Now the quorum is a vote as to how many VGDA's are available in order to
keep the group accessible with an acceptable risk to data integrity should a
volume fail. That is the group remains active if there are still enough
VGDA's and VGSA's available that preserve and acceptable levels of data
integrity. In a two disk system volume group, my notes tell me that the
first physical volume stores two VGDA/VGSA areas and the second only one
area which means if the first drive fails the group becomes inaccessible
(because its breached the majority of the quorum). Therefore I want to
disable the quorum for the rootvg. now I can do that via smit though I am
unsure whether or not I can do this on the fly, or if I have to be in
maintenance, and indeed if I then need to run the bosboot command after such
a change. usually, in the past, I have simply created the mirror with out a
quorum a priori then performed all the bosboot stuff so I have no notes or
experience on changing the quorum of established volume groups on the fly.
particularly the rootvg.

cheers
moth

To disable quorum checking for a two-disk mirrored rootvg setup:

# chvg -Q n rootvg

Then reboot at your convenience for it to take effect.

-Dan

hello dan

thanks for the advice. however do you know exactly what the

QUORUM: 2

line in the lsvg rootvg display refers to?


"Dan Foster" <usenet@evilphb.org> wrote in message
news:slrndsonh7.3f2.usenet@zappy.catbert.org...
> To disable quorum checking for a two-disk mirrored rootvg setup:
>
> # chvg -Q n rootvg
>
> Then reboot at your convenience for it to take effect.
>
> -Dan

In article <dqjt67$c8j$1@news-01.bur.connect.com.au>, rancid moth <rancidmoth@yahoo.com> wrote:
>
> thanks for the advice. however do you know exactly what the
>
> QUORUM: 2
>
> line in the lsvg rootvg display refers to?

It means that two drives are required to achieve quorum.

If one drive fails while this is set to 2, the system will intentionally
panic, because quorum requires 1/2 of all drives + 1.

Sample quorum calculation:

2 drive setup: 1 drive fail, quorum requirement is 1/2 of 2.. that's 1
drive. Plus one drive. So, 2 drives is required to maintain quorum.

If only 1 is working, system automatically panics because 1 working
drive is less than minimum quorum required of 2 working drives.

To correct that, you need to do the two steps mentioned earlier.

The quorum mechanism is designed to prevent potential data corruption or
loss, but it does not make much sense in a two drive mirrored setup.

So the standard advice is to disable quorum if you have that setup --
which you do.

-Dan

ah ha. so i was initially correct (see original post). the confusion was
from where the 2 came from as in, was it being specified somewhere manually.

i know that for a two disk system when one fails the mirrored vg goes down
due to the assignment of VGDA's over the pv's (see my original post). Thus
all that QUORUM: 2
line is saying is that quorum checking has been enabled and because of that,
2 disks are requred for quorum (given the situation of haveing only two
drives in the volume group). if it were a three disk system then i would
guess that the line would be something like QUORUM 1. for in a three disk
system each disk has its own VGDA and there is one set of these on every
drive thus losing one drive still gives 66% of all VGDA's available and thus
51% of the quorum is met.

silly - i should have seen that.

cheers
moth



"Dan Foster" <usenet@evilphb.org> wrote in message
news:slrndsquee.2tp.usenet@zappy.catbert.org...
> In article <dqjt67$c8j$1@news-01.bur.connect.com.au>, rancid moth
> <rancidmoth@yahoo.com> wrote:
>>
>> thanks for the advice. however do you know exactly what the
>>
>> QUORUM: 2
>>
>> line in the lsvg rootvg display refers to?
>
> It means that two drives are required to achieve quorum.
>
> If one drive fails while this is set to 2, the system will intentionally
> panic, because quorum requires 1/2 of all drives + 1.
>
> Sample quorum calculation:
>
> 2 drive setup: 1 drive fail, quorum requirement is 1/2 of 2.. that's 1
> drive. Plus one drive. So, 2 drives is required to maintain quorum.
>
> If only 1 is working, system automatically panics because 1 working
> drive is less than minimum quorum required of 2 working drives.
>
> To correct that, you need to do the two steps mentioned earlier.
>
> The quorum mechanism is designed to prevent potential data corruption or
> loss, but it does not make much sense in a two drive mirrored setup.
>
> So the standard advice is to disable quorum if you have that setup --
> which you do.
>
> -Dan