Dell's latest update to the Compellent Storage Center includes a novel approach to tiered SSDs, but it comes with caveats
Software licensing add-ons are available for per-array features such as FastTrack, Remote Instant Replay (remote replication), and Live Volume (interarray volume migration). These are available in base licenses that cover 16 active disks (excluding hot spares and such) and add-on packs that will license an additional eight. It's not hard to see that this is one area in which tiered flash configurations (which feature fewer disks versus 15K configurations) can show a clear savings.
Array management is provided by a mix of the Storage Center software, resident on each set of controllers, and Enterprise Manager, which forms the single pane of glass necessary to orchestrate the operations of multiple arrays. Strictly speaking, while you can live without Enterprise Manager (a separate Windows-based, SQL-driven reporting and management app), it does make simple tasks such as configuring a volume to replicate from one array to another substantially easier. You'll also need Enterprise Manager if you intend to integrate with third-party software such as Microsoft's Virtual Machine Manager or VMware's Site Recovery Manager.
If you already own a Compellent array and are interested in the new flash tiering, you'll have to wait a little while. Although you can purchase a new three-tier array including write-intensive SLC SSDs, read-intensive MLC SSDs, and 7,200-rpm NL-SAS, you can't graft the tiered SSDs into an existing array. Dell plans to add that functionality in the next Storage Center release, so stay tuned on that front. [ Editor's note: Dell's November 21 release of firmware version SC6.4.2 enables in-place upgrades to tiered SSDs. ]
Putting it all together
To sum it all up, I liked the Compellent line before this new release, and I think it has even more potential now. Software features such as Data Progression and Remote Instant Replay coupled with relatively inexpensive, high-performance industry standard hardware combine to deliver a feature-rich and relatively inexpensive primary storage option. The addition of flash tiering and higher-density NL-SAS can only serve to broaden the Compellent's appeal to a wider audience.
However, no enterprise storage on the market today is perfect. The I/O congestion that occurs during snaphshot creation in Compellent's tiered flash configurations under high I/O load really is concerning. Not so much because it exists, but because the management software doesn't expose what is happening when it does.
Certainly, any work Dell can do to eliminate this issue (by tweaking its I/O queuing to further deprioritize tier migrations versus host I/O) would be welcome. However, I think it may actually be more important to do some work on the Enterprise Manager and Storage Center UIs to expose more of what Data Progression is doing. It's not so bad to have the magic stay behind the curtain when it's a once-a-day background task, but when it becomes an always-on feature, having second-by-second visibility into what's happening inside the black box becomes critical.
All that said, I would strongly recommend putting the Compellent Storage Center on your dance card the next time you're facing a major primary storage upgrade.
This story, "Review: Dell Compellent storage delivers flash speeds at disk prices," was originally published at InfoWorld.com. Follow the latest developments in Windows and data center at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.
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