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why should i use bcv devices to expand stripe meta or, can i do everything by myself using clones?
Hi,
i did look at https://community.emc.com/thread/175977 and successfully expanded a bound stripe volume and preserved the data (thanks to all of you )
And i do understand the key is to create bcv devices with the same configuration as source meta "symconfigure -sid 3905 -cmd "add dev 3a0:3a1 to meta 39c, protect_data=TRUE,bcv_meta_head=65d;" - (copied from Sean's answer in the post)
And i also understand its the symm (array) thats estabilishing mirror (local) replication relationship between source meta, given meta head, copied the contents, reconfigured the stripes, copied the data back and removed the relationship (split/detach). Now the bcv can be used as back up or unbound, dissolved and deleted. i have verfied all and works like charm. And its online operation as well as the writes were written to back, which copied back to source before the completion - which is cool
Here is my question:
Say for example, i dont want to use the bcv, create a clone for the source meta, restripe the meta without data protection, synced the data back from the clone i created - is it still same? yes, i understand array did the heavy lifiting for me and same thing by bcv.
but i am trying to understand if i do everything by myself will it give me any added flexibiliy? or its just duplicate work that array did for me for free which is sure to be worked as people have been using for years? is using clone in lieu of bcv gives me any added advantage?
Thank you
seancummins
226 Posts
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September 25th, 2013 17:00
Expanding upon what I meant before --
If you were to do this manually, the steps involved would be something like this:
- Create a clone with the same number of meta members as the source device, and the same striped/concat config as the source device, but with large meta members
- Create and activate a TimeFinder/Clone session between the source and clone device
- Shutdown production host apps, de-provision source device from host, and provision clone device to host
- Rediscover clone dev on host, remount, and restart app
The symconfigure expansion method retains the identity (SymDev and LUN WWN) of the original device, so no outage is required.
Hope that helps...
- Sean
seancummins
226 Posts
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September 25th, 2013 16:00
Hi Dreamer,
It is possible to create clones from smaller to larger devs -- the number of meta members must be the same (each member would be larger, but there would be the same number of members).
The problem you'd likely run into would be the fact that you'd need to take an outage to do this manually, whereas the symconfigure meta expansion method (or whatever the equivalent SMI-S procedure is for it) can be done entirely online.
I can explain more if you'd like, once I'm back online... Hard to do explain in depth from a phone
Thanks,
- Sean
Sent from my iPhone
seancummins
226 Posts
1
September 25th, 2013 17:00
Dreamer,
Assuming you're on VMAX and not DMX, the symconfigure BCV expansion method actually is using clones -- true BCVs don't exist anymore on VMAX. Whenever you use a BCV on VMAX, it transparently uses emulation mode -- i.e. it behaves like a BCV to the end user, but uses clone technology under the covers.
The fact that symconfigure still requires you to use devices that are tagged as BCVs is kind of a legacy thing... but at least it keeps the process consistent across both DMX and VMAX.
Thanks,
- Sean
Dream93
72 Posts
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September 25th, 2013 17:00
Hi,
Yes, Sean - i am using VMAX storage systems and not DMX.
Its good to know that BCVs indeed use cloning technology.
Essentially you are saying that 'symconfigure -sid 3905 -cmd "add dev 3a0:3a1 to meta 39c, protect_data=TRUE,bcv_meta_head=65d;" it does some magic internally which is not exposed by other means, which ensures that there is no outage.
If i do it everything manually without using 'symmconfigure' i loose this as there is no way to achive this oterh than using symmconfigure expansion. Got it.
And one last question, since BCVs are nothing but clones, so i can create a clone, and pass it as BCV - or its mandatory that devices tagged as BCV - in other words in the symmconfigure command can i pass clone as bcv meta head?
Best Regards
Dream93
72 Posts
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September 25th, 2013 17:00
Hi Sean,
Thanks a lot for quick response.
>>> that you'd need to take an outage to do this manually
>>> Shutdown production host apps, de-provision source device from host, and provision clone device to host
>>> Rediscover clone dev on host, remount, and restart app
Cool, thats the difference - with BCV its always online.
But, i have couple of questions though:
but i thought Clone is superior to BCV as its the recent one - https://community.emc.com/thread/88614?start=0&tstart=0
>>> The symconfigure expansion method retains the identity (SymDev and LUN WWN) of the original device, so no outage is required.
Probably i didnt get it - its symmconfigure is doing the magic, so why cant i use the symmconfigure expansion method so that there is no ourage with clone as well? probably its not exposed outside - even though its exposed from symmcli, its probably not exposed in smi-s provider any way.
Best Regards,
seancummins
226 Posts
1
September 25th, 2013 17:00
Dreamer,
That's correct -- the symconfigure "add dev xx to meta" with the protect_data option does have some extra special sauce that is not exposed via other means, and that is the bit that ensures there is no host outage required.
You can manually create clones with or without the BCV tag... but in order to use the symconfigure "add to xx to meta" with the protect_data option, you need to use the BCV tag.
Thanks,
- Sean
Dream93
72 Posts
0
September 25th, 2013 17:00
thats explains all - just FYI, provider does provide a flag (EMC extension) to protect data (EMCPreservedata - to be precise) for bound stripe meta volumes (4.6.1) and it does create bcv with the same configuration and doing all the work flow. i did find some bugs, which opened as opts, but now everything makes sense.
i will change my focus to replication technologies as i think i kind of understand (at least required) details for lun extension.
thank you
Regards.