The high speed of flash storage often makes it easy to justify its high price. But with Dell's new approach to flash tiering, that justification may no longer be necessary. Though the latest release of Dell's Compellent Storage Center includes new hardware goodies such as a very high-density 3.5-inch SAS enclosure and a raft of updates to the array's management and host integration software, the really big news is support for automated tiering between write-optimized SLC (single-layer cell) SSDs and read-optimized MLC (multilayer cell) SSDs.
The Compellent system could already automate tiering between expensive, low-capacity SLC SSDs and spinning disk. However, this new blend of the two predominant SSD techs allows Dell to claim it can deliver an all-flash solution for the price of disk. Using list price as a comparison, the same money that you might have spent on a Compellent array with 72 146GB 15,000-rpm SAS disks will now buy you a similarly licensed array with six 400GB write-optimized SLC SSDs and six 1.6TB read-optimized MLC SSDs.
Even better, that pure SSD configuration can deliver three times the transactional performance (using a TPC-C benchmark), 85 percent less latency, and 15 percent more capacity while consuming 50 percent less power and rack space than its disk-based counterpart. In other words, if you have the right balance of performance and capacity requirements to effectively leverage it, this innovation could save you a wad of cash, deliver a huge performance windfall, or both.
Dell's novel approach to tiering does come with a catch. However, understanding this gotcha and its potential effects in the field requires a deeper understanding of SSDs in general, Dell's Data Progression tiering software in particular, and how Dell has leveraged both in this new release.
A crash course in SSDs
Instead of using mechanical spinning platters to store data magnetically, SSDs use solid-state flash memory to do the job. Although flash memory is used in all kinds of devices from iPods to USB sticks, the kinds you'll find in enterprise storage are typically either single-level cell SSDs or multilevel cell SSDs. The differences between the two boil down to the typical balancing act among performance, capacity, and expense.
Generally speaking, there are two enemies of solid-state storage. The first, generally referred to as "write endurance," is that each cell within an SSD can endure a fairly specific number of so-called program-erase cycles before it will no longer be able to store data accurately. Write-endurance figures are generally reflected in "full device writes per day" -- a metric that gives a user an idea of the overall lifecycle of a device.
The second enemy of solid-state storage, a phenomenon referred to as the "write cliff," is associated with the fact that each cell must undergo a (relatively) time-consuming erasure process before it can be written to. If the background process that erases unallocated cells fails to keep up with the write load the device is experiencing, the device will run out of pre-erased cells, and write performance will fall through the floor.
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