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TimFarren

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MD3220 Disk Capacity Question

Can someone explain how the Dell PowerVault MD3220i uses disk resources?  I have 12x  931 GB (1 TB) disks in the SAN which should equal out to be 11,172 GB but instead the manager says total available to create arrays is 9,138 GB.  Then after the array is created, it takes another 2 disks for overhead?  At the end, the total available resources I can use to create my disk pools is 7,280 GB.  We have 12 more slots available.  What can we expect in terms of being able to expand what we have?  Will we need to create another array?  What's the formula of raw disk capacity to actual usable capacity in these units?  Can I put dissimilar disks in the unit to expand?  Would that force me to create a separate array?

I'm looking for best practice and the best path moving forward to increase the capacity of the SAN.
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Seth Simmons
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depends on your raid setup
if you are getting used capacity of 10 disks, i'm guessing it's a raid 5 with a hot spare or raid 6 which would account for the 2 disk loss from the total raw space

how you carve your raid could depend on what it's used for
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Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)
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TimFarren

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Nobus - thanks for that helpful tidbit.  That's good to know.
The questions at the end of my original post are most important to me.  We want to expand the array by putting 12 more disks inside it.  I'm concerned for the best way to do that.  Best practice - which RAID setup is best (that the MD3200i supports) and why?  We want the best balance between capacity, speed, and redundancy.  Is it best practice to fill the unit with more 1 TB drives, or can I go larger this time and have half filled with 1 TB and the other half with 1.2 TB?  If the latter, would I be required to create 2 arrays, or would the MD be able to do something different?  

Thanks guys.
If you want performance, use all your disks, of the same size in the same array. e.g. a single array.

There is no real best for performance, capacity, and redundancy, all have compromises.

RAID 6, good redundancy, capacity, and performance, but it depends on your requirements, e.g. How many IOPS do you require ?
I don't actually know how my IOPS I need.  I also am not sure how to determine that. We use esxi.  Is there a way to determine or project how many iops are required?
Performance Monitor in the VM, if using database, check vendor applications.

More disks in array will increase IOPS, RAID type can change the IOPS of the entire array.
If I throw more disks at it, can I extend the array without backup and restore?  I don't want to destroy the current contents of the existing array.  Any special considerations?
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