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08-04-2020 07:24 AM
Expand virtual volume - free pool / disk group space - MSA 1050
I've been searching for an answer here and there but didn't find a question close enough to my problem and poor understanding of the subject.
I am looking to expand a virtual volume fromt about 350GB to 2.1TB.
Allthough it looks quite straightforward, something about the free space available is not quite clear.
Here is an overview of the current situation ( currently I cannot add images yet):
Pool B sates 51 GB free, of which 7106 GB unallocated, divided over 2 disk groups.
Each fisk group has 3580 GB free.
I have a virtual volume of 350 GB which I would like to increase to 2.1TB.
I was thinking, there is 7TB free (read unallocated), so that should be ok.
But then I found this in the MSA 1050/2050 SMU reference guide:
Optional: In the Expand By field, enter the size by which to expand the volume. If overcommitting the physical capacity of the system is not allowed, the value cannot exceed the amount of free space in the storage pool.
and since it was not so clear what 'unallocated' meant, I found this:
Unallocated space is space that is designated for a pool but has not yet been allocated by a volume within that pool.
Does this mean that I cannot extend the virtual volume because there is only 51GB left in the pool?
Or if I do so, it would mean that I am overcomissioning space?
Meaning, I will take space that is initally allocated to other volumes ( but not used yet aka free space in the different volumes).
This would also mean that if my extended volume gets full, some other volumes will lack space at some point in time?
Thanks in advance for clarifying and helping me out.
08-05-2020 01:55 AM
Re: Expand virtual volume - free pool / disk group space - MSA 1050
It's very difficult to explain things without watching the problem properly. Let me give a try.
There is something called physical space capacity limit and logical space capacity limit.
First you need to understand what is "overcommit" in MSA. If you enable this option in Pool which means irrespective of physical capacity you are telling your volume can keep on write data till the size you have defined as volume size. This is nothing but Thin volume which starts allocating space when demand of new write comes. So if you have overcommit not enabled at the pool level then whatever physical capacity available in that pool your volume can't cross that boundry. With thin provisioning, the administrator can create a very large volume, up to the maximum size allowed by host operating system where this volume presented. From Storage perspective, The physical capacity limit for a virtual pool is 512 TiB. When overcommit is enabled, the logical capacity limit is 1 PiB.
Hope now it's clear why this line mentioned "Optional: In the Expand By field, enter the size by which to expand the volume. If overcommitting the physical capacity of the system is not allowed, the value cannot exceed the amount of free space in the storage pool."
So if you are enabling overcommit option in the Pool then it doesn't matter Volume size 2TB or 10TB provided your host operating system should support it.
When you consider physical space then you need to check Virtual Disk group only. If 2 VDG shows 7106GB unallocated which means volumes can go upto that much in current condition provided you enabled overcommit and keep increasing volume size to that level or may be more and keep adding more VDG as well.
VDG level allocated means that is the actual data written. Unallocated means volume can write data on those spaces.
So there are many thing to check to give clear idea about capacity calculation (one of them uncommitted space) in MSA Virtual array but hope the above explanation helps you at this moment.
Hope this helps!
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