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VxVM 5.0 – import of DGs from devices cloned on array level

mlukes
Level 3

‎10-12-2008 08:18 AM

Dear experts,

could you explain me in more details how the new feature in VxVM 5.0 – import of DGs from devices cloned on array level (understand EMC BCV, HP BC etc.) on the same server is implemented and how does it work? I know it hasn’t work in VxVM 4.1 (just strange “unsupported” procedure exists).

There is only short message in VxVM AdminGuide that a unique disk identifier (UDID) is added to the disk private region and it is known to DDL (Device Discovery Layer) and both values are compared whenever disk is brought online. But both original as well as cloned disks have to have same UDID saved in their private regions when cloning process finished. Is UDID derived from HW path of the disk (some kind of hash for example)? I think it could work but on the other hand it would complicate import of DG in case of HBA replacement or change in SAN configuration (possible change of HW path) and/or in case of migration to another system.

Does it work in the same way in case when CVM/CFS is used?

Best regards,

Marek Lukes

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1 REPLY 1

Darren_Dunham
Level 6

‎10-13-2008 10:35 AM

My understanding is that it works the same way that you would do it in previous versions.  It's just done internally so that it's safer.  (Very easy to make a mistake when doing it "by hand").  In both cases, the unique identifers are rewritten in the private region.

 

UDID is *not* derived from hardware path.  This allows you to reconnect drives in any order and still do a successful import.  

 

Main issue I can think of that you'd have with this is that it could complicate doing clone restores (if you ever plan to).  The only reason you usually have to do this is to import the clone filesystem on the original server (otherwise the conflict does not arise).  If you're doing that, you're ususally changing the name of the diskgroup as well.  If you did a clone restore, the modified diskgroup would be pushed back to the production cylinders.  Not a dealbreaker in most cases, just something to be aware of.

 

I can't think of any reason that CVM would change how it works. 


--

Darren 

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