Wednesday, 20 December 2006

Modular Storage Products

I’ve read a lot of posts recently on various storage related websites asking for comparisons of modular storage products. By that I’m referring to “dual controller architecture” products such as the HDS AMS, EMC Clariion and HP EVA. The questions come up time and time again, usually comparing IBM to EMC or HP and little comparison to HDS, but lots of people recommending HDS.

So, to be more objective, I’ve started compiling a features comparison of the various models from HDS, IBM, EMC and HP. Before anyone starts, I know there are other vendors out there – 3PAR, Pillar and others come to mind. At some stage, I’ll drag in some comparisons to them too, but to begin with this is simply the “big boys”. The spreadsheet attached is my first attempt. It has a few gaps where I couldn’t determine the comparable data, mainly on whether iSCSI or NAS is a supported option and the obvious problem of performance throughput.

So, from a simple physical perspective, these arrays are pretty simple to compare. EMC and HDS give the highest disk capacity options, EMC, HDS and IBM offer the same maximum levels of cache. Only HDS offers RAID6 (at the moment), most vendors offer a range of disk drives and speeds. Most products offer 4Gb/s front-end connections and there are various options for 2/4Gb/s speeds at the back end.

Choosing a vendor on physical specifications alone is simple using the spreadsheet. However there are plenty of other factors not included here. First, there’s performance. Only IBM (from what I can find) offers their arrays to scrutiny by the Storage Performance Council. Without a consistent testing method, any other figures offered by vendors are completely subjective.

Next, there’s the thorny subject of feature sets. All vendors offer variable LUN sizes, some kind of failover (I think most are active/passive), multiple O/S support, replication and remote copy functionality and so on. Comparing these isn’t simple, though as the implementation of what should be common features can vary widely.

Lastly there’s reliability and the bugs and gotchas that all products have and which the manufacturers don’t document. I’ll pick an example or two; do FC front-end ports share a multiprocessor? If so, what impact does load on one port have on the other shared port? What downtime is required to do maintenance, such as code upgrades? What is the level of SNMP or other management/alerting software?

The last set of issues would prove more difficult to track so I’m working on a consistent set of requirements from a product. In the meantime, I hope the spreadsheet is useful and if anyone can fill the gaps or wants to suggest other comparable mid-range/modular products, let me know.

You can download the spreadsheet here:

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