Recovering from snapshot disk on HSZ controller

[IST Windsor]

We have a mirrored system disk in our storage array on an HSZ80. I would like make a backup of the system disk, prior to a layered product upgrade, by either:

1. splitting the mirrorset to create a single-disk snapshot, or
2. adding a spare disk into the mirrorset and then splitting it off once it has merged in

In both cases I can understand the commands to create the snapshot disk, which are documented in the HSZ manual; I believe I would add a third member to the mirrorset and then 'freeze' it using:

set mirrorset16 nopolicy ! live system disk mirrorset
set mirrorset16 members=3 ! add extra member
set mirrorset16 replace=disk151 ! insert spare into mirrorset
! wait for merge to complete
reduce disk151 ! remove snapshot disk
set mirrorset16 members=2 ! reset nominal membership

But the manual does not explain how to recover the system back to the snapshot disk in the event of failure. What commands would I use in order to copy the snapshot disk's contents back to the live mirrorset, rather than the other way around?

[Uwe Zessin]

Well, first, what you name a 'snapshot' disk is usually called a 'clone' in the HSx world. A snapshot is a pointer-based view.

On all controllers that I am familiar with, the 'REDUCE' command automatically decrements the member count, but you should re-establish the old policy instead.

Dealing with split-mirror clones on the HSG seems a bit risky, because the clone still retains some characteristics of the original and I was able to produce some not-so-nice controller bugchecks during a workshop I was giving :-(
(Yes, I was able to 'fix it life', pheww ;-)
I don't know if the HSZ is more robust.

I don't have all documents handy, but I think the workaround is to write new metadata to the disk:
> initialize DISK151 nodestroy

(is that really a HSZ80? A disk's name usually looks like DISK15100 on a controller that has a wide-SCSI back-end)

To make the old data available, you first need to unmap the old container (MIRRORSET16). You need to find out to which |unit| it is mapped:
> show MIRRORSET16

Let's assume that it is: D0
- document old state:
> show D0
- unmap:
> delete D0
- remap:
> add unit D0 DISK151

I assume that you have no SSP in place, otherwise you have to reconstruct from (show D0).

The old data should be available immediately.

[Uwe Zessin]

Now we have time to restore redundancy...

- document old mirrorset state to list members
> show MIRRORSET16
(Hm, isn't the length restricted to 9 characters?)

Let's assume the members are:
DISK251 and DISK351

- remove old mirrorset
> delete MIRRORSET16

- convert a single-disk unit into a single-member mirrorset:
> mirror DISK151 MIRRORSET16

- choose one member and add back:
> set MIRRORSET16 nopolicy
> set MIRRORSET16 member_ship=2
> set MIRRORSET16 replace=DISK251
> set MIRRORSET16 policy=BEST_?


Well, all this is without the help of any manuals and assuming that the HSZ80 is similar enough to the HSG80 in these details. I hope it makes at least remotely sense to you...

[IST Windsor]

Uwe, thanks. This is definitely an HSZ80 controller, but you're right about the lengths of the names - the mirrors are called MIRRORnn not MIRRORSETnn.

If I understand correctly your commands will make a new mirrorset containing the original snapshot disk, DISK151, and one of the original live disks, DISK251.

In order to return to the original disk configuration, can I bring DISK351 into the mirrorset to create a 3-disk mirror, then drop the original snapshot disk DISK151 out again using:

set mirror16 members=3
set mirror16 replace=disk251
set mirror16 replace=disk351
reduce disk151
set mirror16 members=2
set mirror16 policy=

?

When I make disk151 back into a hot spare do I need to do anything else to it apart from 'add spare'?

[Uwe Zessin · Accepted solution]

>set mirror16 members=3
OK
>set mirror16 replace=disk251
OK
>set mirror16 replace=disk351
OK. Now you need to wait until at least one disk as finished copying

>reduce disk151
>set mirror16 members=2
It is my understanding that the REDUCE command does this for you.

You can check it with:
>show MIRROR16

>set mirror16 policy=
OK.

>add spareset DISK151
should overwrite any old metadata, but you can always preceed it with:

>initialize DISK151

[IST Windsor]

Many thanks, I can try this now with confidence.