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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.
3 Solutions
Seth SimmonsSr. Systems AdministratorCommented:
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
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
What RAID type is this RAID 60, because

12 x 931GB, in RAID 60 is approx - 7.4TB

of which 6.9 is usable.

So you need to work out, what RAID type you require, or add more disks, either to existing RAID array, or create a new RAID array!

More Spindles = More Disks = More Performance = MORE IOPS

in the Array!

Performance versus Resilience, is a compromise, how many disks can you afford to go down, before the RAID set is broken and usable!

If you are uncertain of RAID levels, to select

see here



1 TB = 1000 Gb, as calculated with normal maths; however, for disks, aTB = 1024 GB
so the calculation goes :
1TB = 1000x1000x1000x1000 bytes
in" diskese"  1 TB= 1024x1024x1024 x1024 bytes
makes a lot of difference !
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TimFarrenAuthor Commented:
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.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
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 ?
TimFarrenAuthor Commented:
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?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
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.
TimFarrenAuthor Commented:
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?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Yes you can, but RAID expansion, by adding disks, takes time to add, and performance can be affected, and because this is a disk intensive process, puts strain on the existing disks being added, when the RAID needs striping over new disks.

So it's advisable to have a full backup of VMs, just in case of failure. You should make a backup before any production change to disk storage.
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