# Multiple storage controllers - How to attach the disks



## storvi_net (Jun 27, 2014)

Hello,

I have built a storage server with a single IBM storage controller which I flashed with the LSI-Firmware.
In the future I need additional storage, so I want to add a second vdev (6 disks RaidZ2). For this I would need a second controller.

What are the best practices with multiple controllers and multiple vdevs?

Put 1 vdev per controller
Mix the disks / controller

The second question ist about: How is this problem solved in bigger boxes with > 16 disks?

Thanks
Markus


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## kpa (Jun 27, 2014)

I believe the recommendation is to split the VDEVs so that if one controller breaks totally the pool will be still usable. For 2-disk mirror VDEVs it's obvious how to do that but for RAIDZ you'd need more than two controllers I think.


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## storvi_net (Jun 27, 2014)

This theory is backed by the fact, that if one controller got two vdevs, both vdevs have to be rebuilt, haven't they?


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## phoenix (Jun 30, 2014)

If possible, spread the disks across controllers such that if a controller dies, you still keep redundancy in the pool.

A raidz2 can survive the loss of 2 disks without losing the entire pool.  For a 6-disk raidz2, this means you should not put more than 2 disks onto a single controller.  So, for the best redundancy, you should have 3 controllers, with 2 disks from each vdev on each controller.  That way, if a single controller dies, you only lose 2 disks from each vdev, and the pool remains intact.

Obviously, you are constrained by the hardware.  If you don't have room for the extra controllers, then you make do with what you have.

Our biggest boxes use SAS expanders and put 24 drives onto a single controller.  If that controller dies, we shut down the box and replace the controller and hope everything comes back up.


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## storvi_net (Jul 1, 2014)

So in your opinion it is better to take the risk of leaving the pool online without any backup HDD, than "losing" the pool all at once (sure you have to take care about the whole scenario)?

The thing with the expander is quite interesting - I googled a bit around and it looks nice.

Regards
Markus


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