Two RAID 10 arrays using the same drives

Hello Everyone,

I have this weird situation. I have a Dell PowerEdge R710 server with two PERC H800 cards, the PERC H800 cards are connected to a Dell PowerVault MD1200 Direct Attached Storage. Here is how everything is setup

The first PERC H800 is connected to one of the PowerVault MD1200 EMMs which consists 6x4TB SAS drives

The Second PERC H800 is connected to the other of the PowerVault MD1200 EMMs which also consists 6x4TB SAS drives

I used the Dell Open Manage server administrator to setup RAID 10 with intelligent mirroring on both the cards. When choosing the drives I chose all the drives. After the RAIDs were initialized I realized that virtual drives showed a total capacity of 24 TB on both cards. Further investigation revealed that the same drives were assigned to both the RAIDs. I have a total of 48 TB available to me on both RAID 10s, the total capacity of the drives is also 48 TB. Now I understand that RAID 10 halves the total capacity of the drives, then how is that I have the total capacity available to me (24 TB on each RAID 10) and how is it that both RAIDs are using the same drives (drives with same serial numbers part of both RAID 10s)? is that a benefit of the intelligent mirroring feature, or is it just a glitch. I will appreciate your help on this. Thanks!
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NewAvenuesAsked:
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arnoldCommented:
This is called. Multipath if one h800 or the cable or the MD enclosure controller fails you will still have access to the single 12TB storage volume.
Though not sure your setup is appropriate

The vault is often setup for clustered, have you checked with Dell dealing with what your needs are and your setup?
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NewAvenuesAuthor Commented:
Thanks for commenting on this Arnold. I work for non-profit, we can't really afford new equipment so I purchased used equipment except the hard drives, hence don't have access to Dell support. I did post this question in the dell support forum but no one has responded yet.

There must be something wrong with my setup, because I have two drives available to me in the OS, I created test files on them, each RAID 10 is giving me about 22 TB virtual drive, so I think if a card or cable or enclosure were to fail I will loose the entire drive.  Also I configured the RAIDs differently, one has 64KB stripe size, the other has 1 MB stripe size.

I think what's really strange is that both the RAIDs are using the same physical drives. When I look at the physical drives that make up the virtual drive, I see 12x4TB drives assigned to first RAID and then the same 12x4TB physical drives assigned to the second RAID.

This system is not in production yet, may be I will pull out a physical drive and see what happens.

I am willing to give access to the server through teamviewer if anyone wants to look at the configuration.
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arnoldCommented:
The MD1200 is built to be chained, meaning one connection from a server, the other is used to connect a sevond enclosure.

The issue with your situation is that you might run into file collision one overwriting the data from the other.

I.e. only on reboot both will reflect the same files.
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arnoldCommented:
Often the two connections are used by individual hosts in a cluster configuration, meaning inky ine host can write to the array at a time.
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arnoldCommented:
Here is the write-up

http://www.dell.com/downloads/global/products/pvaul/en/storage-powervault-md1200-specsheet.pdf

You can have in each port of the H800 up to four chained MD12xx enclosures.
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Jim_NimSenior EngineerCommented:
It sounds like you're saying you have a single MD1200 enclosure with a total of 12 x  4TB drives, and you're operating in split-bus mode so that each EMM has access to 6 of the drives... or at least that's what you're intending to do. If each controller is accessing the same drives, then you may not have the split-bus mode switch on the front of the enclosure set correctly. If you try to go forward with your current setup, you'll likely see data corruption on the disks as the two PERC controllers fight over who is "in control" over the drives, and they'll continually overwrite each others' configuration data on the disks.

The problem here is NOT related to multipath IO.

However... split-bus mode isn't really intended for multiple controllers in the same server, it's aimed at use with multiple servers. Since these are 7.2K drives, you won't really see any performance benefit from using 2 controllers as opposed to 1, and the two controllers can't provide you any redundancy with this configuration.

If you stick with your current setup and get the split-bus mode switch set correctly (in the down position), then each controller card should only see 6 drives. You should also only have each controller connected to one EMM via a single cable.

Another option which would provide more redundancy would be connecting a single controller to both EMMs, with the enclosure set to "unified" mode (split-bus switch set "up"). This gives the single H800 controller access to all 12 drives through two redundant paths, and if an EMM fails you won't lose access to your data. You would be dealing with MPIO in this setup, but this only requires the proper driver for the controller is installed, nothing more.

So your current setup (if you get it functioning) would lose access to 6 disks if an EMM, H800, or cable fails. The setup I recommended would keep data access if an EMM or cable fails, but lose access to ALL data if the H800 fails.
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NewAvenuesAuthor Commented:
Hello Guys,

Jim_Nim, I didn't even know about that little switch, I switched down but when I turned the server back on I realized that only drives in one enclosure were on. This is when I recalled Arnold's comment about daisy chaining the MD1200, so I looked at the back of the device and realized to my embarrassment that I connected one controller to the IN connector and the other to the OUT on the MD1200. The MD1200 is placed in a way that it is hard to see that there are other ports to connect to. Anyhow I connected both controllers to the IN ports of the MD1200, and now I have six drives available on each controller.

I want to use the MD1200 with the R710 server purely as a storage device, not for clustering. The reason I installed two controllers is for redundancy not performance, they are used controllers, they don't have warranties, I am less worried if I have two controllers, because if one fails I can just move the drives over to the second connector of the working controller and import the configuration, the second controller will run the drives while I go looking for another used replacement. Do you guys see any issue with this setup?

I've setup RAID 10 on both controllers with 6x4TB drives. One will be used for general file storage (documents, pics, etc) so I configured it with 64KB stripe element size. The other will be used for onsite backup, since backup files and system images are large files I configured it with 1 MB stripe element size.

I am curious to know what you think about my setup and if there is anything that you think I should be concerned about. Really appreciate your help.

Thanks!!!!
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NewAvenuesAuthor Commented:
The problem I was experiencing was due to reconfiguration on my part. The knowledge and expertise of the experts that helped me led me to discover my error. Thanks!
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