the_san_man
2 Iron

Striped meta protected expansion - When is the additional capacity available for host I/O?

I've searched the KB and forums and haven't been able to find the answer.  I have periods where I do a considerable amount of meta expansions, usually leading up to Q4.  In some cases, the additional space is needed quickly.  I would usually wait for the entire expansion process to complete before extending the volume on the host.

Recently however, I had a host that I was extending multiple LUNs on.  After the first one LUN expansion was complete, I went ahead and started the second. By the time I got on the host to expand the first volume, both physical disks showed to have the new space available. That made me curious so when I expanded another LUN, I checked to see when the host discovers the additional space.  At roughly step 60 of the script, the host begins seeing the space.

My question is, is it safe to go ahead and expand the host volume at this point? This is on a DMX4, 5773.160. The same thing occurs on my VMax, but that's not usually a concern since the VMax thin meta expansion is so quick anyway.

Thanks in advance for your time and thoughts.

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6 Replies
Amita2
3 Argentum

Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

Hi,

You should always wait till the script actually finishes before starting expand on the Host Side as there could be still VTOC running for the device in the box.

Regards

Amita Wasson

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AshokaReddy
2 Bronze

Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

Hi,   
The Newly added space will be available to the host as soon as you commit the config change to expand the meta. The meta expansion is a backend process and host will not have the knowledge about this. Symmetrix will manage the incoming writes coming in using the BCV devices.

Regards
Ashoka Reddy

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the_san_man
2 Iron

Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

So you're saying that even though I have comitted the config, and it is still running (say it's on step 61 of 74), I can safely extend the volume? I would expect that to be the case but I wasn't entirely sure and never attempted it. That's contradictory to what Amita said above though.

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Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

Ashok,

          Are the BCV devices used for managing the new incoming host writes? I thought it was to prevent the data loss during the restriping activity. So, if BCV is not used for supporting the host writes, in that case, can we still access the meta as soon as we commit?

Thanks,

SreeHari Karanam

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sauravrohilla
4 Tellurium

Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

Hello All,

Just want to clear some doubts on expanding meta. When you expand the meta using BCV (or not using BCVs), the extended space will ONLY be available once the commit is finished and you get the message "The configuration change session has successfully completed".

Once the extend is completed from the host, you need to run the rescan on the host and make sure you see the extended space in the disk management. In server 2008, you can right click on the disk you want to extend and click extend.

Hope it clears some of the doubts..

regards,

Saurabh

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RobertDudley
3 Zinc

Re: Striped meta protected expansion - When is the additional capacity available for host I/O?

What you may want to look at is what is the safe thing to do here.  If the script is not complete and you are able to "see" the additional space from your server do you want to really start using it yet?  If you do and the script has an issue and bombs out now where is your data at?  Is your newly written data that exists on the BCV's still available or are you back on the "old" data luns. 

The safe course of action is to wait until the script is completed and you then are on the newly expanded meta volumes versus the more aggressive action of claiming the space prior to script completion.

Where would your systems be if you had a script issue and lost a chunk of your data because you didnt wait.....