Two RAID 6 logical disks: 75GB for host OS and balance to guest data

I have a question about a RAID6 configuration with (8) 600GB 10K RPM drives on a Dell PE330.  The T330 chassis can accommodate up to (8) drives.

Phillip Elder suggested the following to a previous question titled "VM 101 Question!"  Posted on 2017-11-15:

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by:Philip Elder
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Technical Architect - HA/Compute/Storage
ID: 42386217
2017-11-29
RAID 6 is just fine for a virtualization stack. Many questions answered here: Some Hyper-V Hardware & Software Best Practices.

Two RAID 6 logical disks: 75GB for host OS and balance to guest data (RAID 6 6x 900GB = ~3.5TB)

Once the host OS is in set up the second partition and once the Hyper-V Role is set up make sure to set the configuration and VHDX file setting to that partition.

I believe in order to setup each RAID6 logical volume it requires at least (4) drives.  So, how does the above recommendation make sense.  I am just trying to understand the premise.

Are we saying, setup  each RAIbD6 logical array with (4) 600Gbs disks which would provide approximately 1.2TB of storage each.  That does not make much sense to me if that is the case.  Placing hyper-v os on the first logical disk use no more than 100Gbs of storage.  Trying to make sense of this response.
 
I looked at the following article about RAID6 logical disks:  

https://storage.microsemi.com/en-us/support/infocenter/release-2015-1/index.jsp?topic=/maxview_ug.xml/Topics/RAID_6_Arrays.html

Can you please breakdown your response.

Thank you.
cmp119IT ManagerAsked:
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Don S.Commented:
Don't confuse logical disks with physical disks.  You can have any number of physical disks in a RAID 6 array as long as there are 4 or more.
You can have several logical disks configured on a Raid 6 Array volume.  What we are saying is to create a separate Volume to hold the Hyper-V host partition.  Create a second volume and/or array to hold the Guest Storage.
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cmp119IT ManagerAuthor Commented:
So that I am clear, create one large RAID6 array (8 disks) usable storage approx 3.6TB.  When installing the hypervisor create a 100Gb active partition, and create an extended partition for the remain RAID6 disk space.  All of this would be done during the Windows server install, and then afterwards create a second disk partition to store the VHXD files.
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Philip ElderTechnical Architect - HA/Compute/StorageCommented:
When in the RAID controller's BIOS we set up two RAID 6 logical disks. The first is the 75GB for the Host OS and is set to Bootable.

The second RAID 6 logical disk is the balance of the space available in the array.

It's actually quite simple once in to the RAID BIOS. The UI is a bit clunky but it works.

EDIT: Note, we do this setup at the RAID BIOS level not the OS install level. At the OS install level one gets MBR so anything beyond 2TB is not accessible.
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cmp119IT ManagerAuthor Commented:
I am on the phone with Dell Support and they stated slicing the array is not recommended anymore because they have seen if one logical disk within a single large array causes other configured logical disks to go down as well.  They say that use to be recommended, but does not apply today.  

Dell stated to keep the hyper-v os on a separate array (RAID1), and then create  a separate RAID6 array with the remaining 6 disks.
This is what I've done with all of my physical and virtual servers until this server purchase.

I really would hate the idea having two 600gb drives in RAID1 for the host os.
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Philip ElderTechnical Architect - HA/Compute/StorageCommented:
That's news to me.

Setting up two different arrays on one adapter will actually have a detrimental performance impact.

We've been doing RAID 6 set up with two logical disks since I can remember. Other than firmware issues either on the disks or the RAID controller itself, we've not seen any issues with having two logical disks on one array.

I'd like to see the documentation that says what they said.
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DavidPresidentCommented:
You should really just do a single RAID1 with 2 disks, then a 4-drive RAID6 for data.  The RAID1 can be for o/s, swap, index files, scratch table space, and anything write intensive.   You can then also back up the O/S to the RAID6 [Dont confuse backup with offsite archiving].   This gives best of all worlds.
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cmp119IT ManagerAuthor Commented:
David - yes, that's what I plan on doing.  That's the way I've setup all my existing physical and hyper-v servers.  I usually purchase two smaller hard drives for the hypervisor os (RAID1), and then larger drives for the vhdx file on a separate RAID6.  

I also need to include, I deviated from my normal practice of purchasing server hard drives as usual.  Meaning, it was suggested in another question I posted that 15K RPM SAS hard drives are not as efficient/fast/deprecated as comparable 10 RPM SAS hard drives.  I asked Dell Lead Support about that as well, and they state this statement is not true, and if both drives are SAS 12Gps then the 15K RPM will notably be faster than the 10k RPM drives.  I am pretty much over this though.  

Going back and forth about drives and RAID array setup has caused more heartache and doubt than I cared to deal with.  I do not mean to take offense, so my response is not intended that way.
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DavidPresidentCommented:
Dont generalize 15K vs 10K performance.   Just read the specs.   The 12g vs 6g is irrelevant for a mechanical drive attached DIRECTLY to a RAID controller A single disk cant possibly saturate the 6G pipe, so if the disk is direct attached then 6G = 12G performance.  [Note, if using a 12G expander, then of course use 12G disks.  Also you are in a VM so I/o will be cached by hypervisor so you won't be doing any 4K i/Os, and will be doing much larger I/Os due to the aggregation and larger block size that the hypervisor automatically does for  you]

You need to look at latency time and how many IOPs.   For the RAID1 disks, you'll do more random I/O.   The RAID6 will be more sequential I/O.
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