VMware Storage Part 8: Storage Networking Revisited

Storage networking is a topic that could easily descent into deep religious debate, but I often get questions from customers and partners such as what does it matter in a virtualized environment, if we are virtualizing, why should we care what the storage network looks like. The specific question more recently was around 1GbE iSCSI versus SAS, so I want to specifically address the SMB market space, but the decision points are not dissimilar.

To start with a quick look at the SAS protocol. SNIA has a great presentation on the differences between the various storage networking protocols, http://www.snia.org/sites/default/education/tutorials/2011/spring/storman/GibbonsTerry_Shareable_Storage_With_Switched_SASv2.pdf. SAS, as it points out, is not a standard, but rather a way of conveying unique sas attributes. This is yet another way, primarily in highly dense server scenarios, to present shared storage. Essentially it is a way of sharing out direct attached storage. The main draw here is the speed over the 1GbE iSCSI. Since SAS is generally at 6Gbps, and can run over 4 channels for 24Gbps.

The main challenges for SAS focus on deployment and cost. It is often looked at as a cost saving measure over Fibre Channel, high speed and a lower cost. The challange here is that it is fairly limited in it’s scalability. It also introduces some complexity not found in iSCSI. Bringing in new switches, and zoning them is reminiscent of fibre channel, which is far more scalable.

iSCSI is not without it’s challenges of course. There is the consideration of using separate physically isolated switches from the remainder of the network, or using VLAN tagging on existing switches. 1GbE iSCSI can be saturated given enough utilization, and proper design is critical to minimize latency.

So to answer the question, what does it matter, the first response is is it supported. VMware publishes an exceptional Hardware Compatibility List, http://www.vmware.com/resources/compatibility/search.php, which should always be the first stop on these decisions. Secondarily to that, know your environment. While Switches SAS does have it’s place, at this point, in the SMB environment, it often makes sense to stick with what is a known quantity. Every environment already has an IP network, so leveraging that, or extending it is the simplest way of moving forward. This keeps the environment standards based, and does not require sticking with a specific solution. At the end of the day, beyond what is supported, the best design principle is to keep everything simple. While it may not matter as long as it is supported, generally speaking, then best designs are the ones which are well documented, easily repeated, and simple.

As always, there are exceptions to every rule, but I would say that using iSCSI is preferable over SAS for all those reasons, why make things more difficult than they need to be.

VMware Storage Part 8: Storage Networking Revisited

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