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I'm trying to figure out how EQL works with multiple controllers. Dell sales people recommended having one RAID array on one EQL unit. But will this set up allow utilizing both controllers? Or will the throughput be limited to 2G, with IOPS limit of a single controller in that setup?

I know that for MD3000i we were advised to set up 2 RAID arrays, so that primary path can be set in a way that will utilize both controllers, and all 4 of the NICs.

So how's it done with EQL?

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I'm not 100% sure, but I was under the impression that the other controller just sits there as a failover unit. At least, that's how ours are configured, perhaps there are other methods... – Mark Henderson Oct 24 '11 at 23:05
up vote 6 down vote accepted

The second controller is always in standby mode. There is not a way to make both controllers on an Equallogic SAN active at the same time.

The purpose of the two controllers is to provide failover -- not to increase performance. This is actually desirable. If you think about it this method provides you with the only way that you can maintain your preformance, or SLA levels, in the event of a controller failure. If you were using both controllers at the same time, with high demand on both, and one failed, you would be asking one controller to do 200% of work, which is not possible.

It sounds wasteful I know, but this is how they are designed to work.

Therefore creating addtional RAID sets, or volumes on the same SAN will not engage the standby controller.

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I can second this - we have a number of Equalogic units and the 2nd controller is for availability, not throughput. If you need high throughput then they do a controller for at least some of their arrays that supports 10Gb interfaces. We're using that here and it works pretty well. – RobM Oct 25 '11 at 7:39
Thirded. The second controller should be connected in an identical way to the first, that way should the first controller fail the second controller will pick straight back up without your servers really noticing the difference. You can test this by "restarting" the equallogic in the GUI. The restart button actually restarts the controller, causing the other controller to pick up the slack instantly. N.B be sure to enable portfast on your switches before trying this otherwise you'll have 10 seconds downtime. – Alex Berry Oct 25 '11 at 11:51
When making your decision between which storage array you want, you can ask for one with active/active controllers and simply deal with the performance hit if whenever one is down. – Basil Oct 25 '11 at 12:57
Hmm, strange. I understand about performance hit. Degraded anything is not a fun thing to have. But the hope is that you'll have it very rarely. We use MD3000i in one other setup, and we have 2 RAID arrays, with primary path on 2 different controllers. That way we can use both when things are OK, and suffer through if one controller goes bust. Seems to be a questionable move to remove this feature from a higher end product. – Sergey Oct 25 '11 at 15:30
Confirmed with Dell rep – Sergey Oct 25 '11 at 17:45

Have a look at the EqualLogic Configuration Guide. It explains in gory detail how active/standby controller failover works and provides best practice connection diagrams.

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