Redundancy On The Dell MD3220i - How?

[parityboy]

I notice that the MD3220i - being an iSCSI SAN box - has dual controllers. Obviously this is for fault tolerance, but how exactly is this achieved?

Assuming that two H700s are connected to the same backplane, what causes the system to failover to the second controller? Is it a feature in the software/firmware? I'm asking because I'm wondering if I can achieve the same with a 12-bay R515 and something like NexentaStor or QuantaStor.

Many thanks.

 

[mwroobel]

You need to install the multipath drivers and related software (MDSM). You can read more information specific to your hardware here.

 

[Billy_nnn]

I notice that the MD3220i - being an iSCSI SAN box - has dual controllers. Obviously this is for fault tolerance, but how exactly is this achieved?

Assuming that two H700s are connected to the same backplane, what causes the system to failover to the second controller? Is it a feature in the software/firmware? I'm asking because I'm wondering if I can achieve the same with a 12-bay R515 and something like NexentaStor or QuantaStor.

Many thanks.

It's not really the same thing TBH - the 3220i basically has two independent "systems" (each purpose designed purely as an iscsi server) in the enclosure (they just call them controllers for simplicity) - the idea being that if one "system" fails, you can maintain access to your storage via the other.

The R515 is a single server with 8 or 12 drives inside - you can of course offer it's storage out over the network via iscsi, just like the 3220i, but you only have one system (or iscsi controller if you prefer), so if that fails your storage disappears from the network - there is nothing to failover to (unlike the 3220i).

 

[parityboy]

@Billy_nnn

So do the RAID controllers in the MD3220i connect to a common backplane? Also, when you create a RAID on the MD3220i, what does the secondary controller do? Simply import the config? Obviously, both RAID controllers cannot create/destroy arrays simultaneously, right?

Or have they implemented what is basically software RAID with two HBAs which have a big RAM cache on each of them, and then use multipath drivers to control the HBAs, as mentioned by mwroobel?

 

[kac77]

@Billy_nnn

So do the RAID controllers in the MD3220i connect to a common backplane? Also, when you create a RAID on the MD3220i, what does the secondary controller do? Simply import the config? Obviously, both RAID controllers cannot create/destroy arrays simultaneously, right?

Or have they implemented what is basically software RAID with two HBAs which have a big RAM cache on each of them, and then use multipath drivers to control the HBAs, as mentioned by mwroobel?

If it's anything like the other Dell PowerVaults I've worked on it relies on Windows Server Cluster Services. You have two servers both plugged into the storage array. You have a primary and a secondary. There's a heartbeat between the secondary and the primary. When the primary fails the secondary server takes over. The Storage Array will only allow one server to communicate at a time.

 

[Billy_nnn]

So do the RAID controllers in the MD3220i connect to a common backplane?

Normally yes, but I'm not that familiar with the MD3220i that I can tell you exactly how it's backplane has been designed though!
However, in such arrays the backplane is usually a very simple affair, with little other than connectors on it - the disk connectors on one side, and the power and controller connectors etc on the other. There's little to go wrong usually, other than broken connectors.

Also, when you create a RAID on the MD3220i, what does the secondary controller do? Simply import the config? Obviously, both RAID controllers cannot create/destroy arrays simultaneously, right?

The two controllers will be in constant communication - when you create a LUN on one, the other knows about it pretty much instantly. While the two controllers are independent in the sense that each can run the whole array on it's own (in the case of failure of the partner controller), in normal use they work together very closely.

Or have they implemented what is basically software RAID with two HBAs which have a big RAM cache on each of them, and then use multipath drivers to control the HBAs, as mentioned by mwroobel?

It's a little more than that TBH. The controllers also mirror each other's write cache data.
I suppose in concept it's a little like a 2 node cluster, with a disk array attached to (and accessible by) both nodes. The nodes offer out the iscsi LUNs in a high availability framework - ie if one node fails, the other takes over it's LUNs and offers them out itself.


The multipathing software mwroobel referred to is for installation on hosts which attach to LUNs on the array. It then manages any path failovers on the host, which might need to done in the event of an array controller failure etc.



As to your original question
"I'm wondering if I can achieve the same with a 12-bay R515 and something like NexentaStor or QuantaStor".

The answer is no - at least not with a single R515 anyway.

You could create a config broadly similar - you'd need two servers though, a JBOD disk array, and an cluster capable OS (eg Nexenta).
You'd have to decide whether it was worth it though - I suppose it'd come down to your needs, and then cost and supportability.

 

[parityboy]

@Billy_nnn

Nice explanation. Thank you.