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Shadowed disk

Willem Grooters
Honored Contributor

Shadowed disk

Just curious, if anyone has ever looked into this:

If I created a shadow set DSA0 consisting of DISKA and DISKB, will DISKA and DISKB be phyiscally equal, or logically?
In other words: if a file is badly fragmented on DISKA, will it be as bad on DISKB (physically equal) or could it be be less fragmented on DISKB (logically equal)

Willem Grooters
OpenVMS Developer & System Manager
12 REPLIES
Duncan Morris
Honored Contributor

Re: Shadowed disk

As far as I am aware, the shadowing copies by LBNs rather than logical entities. So, in your hypothetical case the file would be as badly fragmented on DISB as it is on DISKA.

Karl Rohwedder
Honored Contributor

Re: Shadowed disk

I suppose, the disks will be equal in terms of LBN, otherwise all that mini-merging etc.
would be very difficult.

regards Kalle
Uwe Zessin
Honored Contributor

Re: Shadowed disk

On a 'steady state' shadow set, the disks have the same content right down to the block level. So, yes, a file *IS* fragmented on BOTH disks.
.
Jan van den Ende
Honored Contributor

Re: Shadowed disk

Willem,

The members are equal, block for block, bit for bit.
Even the UNallocated blocks are equal.

hth

Proost.

Have one on me.

jpe
Don't rust yours pelled jacker to fine doll missed aches.
Robert Gezelter
Honored Contributor

Re: Shadowed disk

Willem,

Shadowing is LBN by LBN. That means the drives will be equal at the fragmentation level). On DSA disks, bad block handling is virtualized BELOW the LBN level, so you will not see those appear.

- Bob Gezelter, http://www.rlgsc.com
Heinz W Genhart
Honored Contributor

Re: Shadowed disk

Hi Willem

I aggree with the issues made in the above responses.
We had once a problem with shadow sets. This time we got a macro program, which compares two shadowset members and reports on any LBNs which do not contain exactly the same data.

If you are interested to get this program please let me know. I can post the source of this program here

Regards

Heinz
Uwe Zessin
Honored Contributor

Re: Shadowed disk

Yes, that was a bug in the shadowing code.
You could also get a program from HP service.

These days, I think you can use
$ set shadow /demand_merge device:
.
Willem Grooters
Honored Contributor

Re: Shadowed disk

I thought so, deducting it's feasable - if not required. It would be nice (and handy) though if the equality would be logical - for defragmenting on-the-fly, for instance.
Pity it cannot be done that way :)

Willem Grooters
OpenVMS Developer & System Manager
Volker Halle
Honored Contributor

Re: Shadowed disk

re: Heinz,

$ ANAL/DISK/SHADOW is available in OpenVMS V7.3-2 to do shadowset block-by-block comparision between all the shadowset members.

Volker.
Robert Brooks_1
Honored Contributor

Re: Shadowed disk

jpe wrote . . .

The members are equal, block for block, bit for bit.
Even the UNallocated blocks are equal.

---

That is not necessarily true. Please see the
help for INIT /SHADOW for the details, specifically the discussion about the use of the /ERASE qualifier in order to reduce the potential time of a full merge operation.

-- Rob
Jan van den Ende
Honored Contributor

Re: Shadowed disk

Robert,

yes, I was aware of that. If I had had some more time when making my entry I even would have hinted at that. But as this only is a new feature, and even then, best used WITH erase, I chose to not mention that.
Even if INIT/SHAD is used without /ERASE, every block that ever gets used is made identical, to (if things go well) never get out of synch again.

Proost.

Have one on me.

jpe
Don't rust yours pelled jacker to fine doll missed aches.
Jan van den Ende
Honored Contributor

Re: Shadowed disk

Willem,

from your Forum Profile:


I have assigned points to 642 of 686 responses to my questions.

Some of the unassigned answers date back to 2004.

Maybe you can find some time to do some assigning?

http://forums1.itrc.hp.com/service/forums/helptips.do?#33

Mind, I do NOT say you necessarily need to give lots of points. It is fully up to _YOU_ to decide how many. If you consider an answer is not deserving any points, you can also assign 0 ( = zero ) points, and then that answer will no longer be counted as unassigned.
Consider, that every poster took at least the trouble of posting for you!

To easily find your streams with unassigned points, click your own name somewhere.
This will bring up your profile.
Near the bottom of that page, under the caption "My Question(s)" you will find "questions or topics with unassigned points " Clicking that will give all, and only, your questions that still have unassigned postings.

Thanks on behalf of your Forum colleagues.

PS. - nothing personal in this. I try to post it to everyone with this kind of assignment ratio in this forum. If you have received a posting like this before - please do not take offence - none is intended!

Proost.

Have one on me.

jpe
Don't rust yours pelled jacker to fine doll missed aches.