Replacing a Quorum Disk

[Wim Van den Wyngaert]

Eberhard & co

Shadowing the q disk is not allowed for the case that the 2 disks would start to live separately.

But mirroring is allowed because the controller hides the fact from VMS.

So, shadowing 2 disks behind 1 (dual) controller should also be possible.

FWIW

Wim

[comarow]

They need to send me an email as it's a propriatary document. My email is on my profile. It was an old STARs article.
robert.comarow@hp.com

[Ian Miller.]

old STARS articles are available e.g.
http://h18000.www1.hp.com/support/asktima/operating_systems/CHAMP_SRC930821000052.html

[Eberhard Wacker]

Wim, for what's it worth ?!

I simply wish to use a quorum disk as part of a shadowset just to be able to use this shadowset for more than quorum.dat, pagefiles etc. (and in general I prefer a quorum node).

If the quorum disk is defined all over the cluster as 1 dedicated member of a shadowset, then the case can not occur that this important part of a clustered system starts to live separately. I know, this would mean change of basic vms code as I've mentioned before, but this is another story.

And a last remark: sure, we use mirroring for our quorum disks ...

Cheers,
EW

[Richard W Hunt]

You should not mirror a quorum disk if that is ALL that it is.

BUT if you use it for other purposes, the other purposes will govern whether you mirror it. If it is a SWAP/PAGE disk (ONLY) then you STILL should not mirror it. You are wasting WRITE operations on the mirror.

But if you are using a shared system disk, that disk can be the quorum disk. Ask yourself the purpose of a quorum disk. It is to prevent the system from coming up if you don't have enough to make it work. Well, if you are on a cluster with an even number of physical members AND the system disk is shared, it is the best candidate, bar none. 'cause if you don't have the shared system disk, you are SO hosed...

Now if you have distinct system disks, this isn't true. But if you want to do this "right" then consider some applications disk without which you shouldn't be running your system. Like if you have a separate disk for user home directories, make THAT you quorum disk. QUORUM.DAT doesn't really contain much data anyway.

[comarow]

No, you should not mirror a quroum disk.
It is possible each part of the cluster will see another part of the quorum disk and you'll partition the cluster.

That's why shadow disks are not supported for clustering.

As far as building quorum disk, the trick is it doesn't use the quorum disk until after you boot. So boot once with enough votes to build the quorum.dat file.

example set votes 3, expected 4 and qdskvotes 1 for 1 vote, then reset it back to 1 vote.

good luck. It's easy

[Richard W Hunt]

good point, comarow

Host-shadowed system disks should not be used for quorum disks. Controller-mirrored disks, since they are mirrored even when the system isn't "up," are viable candidates.

[Robert Brooks_1]

With respect to quorum disks and host-based volume shadowing -- you CANNOT use either
a shadow set virtual unit or a shadow set member device as a quorum disk.

Statements like "you should not do this" are somewhat irrelevant, since you CANNOT do this.

This restriction is enforced in the code. If you attempt to violate this, you'll see the error message

\%CSP-E-QDSHADOW, Quorum disk may not be a shadow set or a shadow set member\

(For those with access to the source listings, this is found in module [SYSLOA]CSPQUORUM)

You can, of course, use a controller-based mirrored set as a quorum disk.

Some of our more paranoid customers use mirrorsets as shadow set members for important devices.

[Jan van den Ende]

That DOES raise an interesting question.

I am not well enough into storage, but _IS_ it possible nowadays to create multi-site _MIRROR_ sets?
Because that _WOULD_ open up the possibility of configurations which are so rightly prohibited by not allowing to shadow quorum disks.

-- and I happen to know just the person to immedeately implement it!

Uwe, maybe this fall in your area of expertise?

Proost.

Have one on me.

jpe

[Uwe Zessin]

Sure, any half-way competent in-band virtualization appliance can mirror between two different storage arrays that can be located in two different data centers. They are true mirrors and not a 'replication' implementation like DRM/CA.

Another way is the XIOTECH Magnitude storage arrays. There is a configuration that puts both controllers into different data centers. Last time I checked, the back-end connects via FC_AL to the controllers and can be distributed as well.

OpenVMS support... well that's a different thing.