Review: Dell Compellent storage delivers flash speeds at disk prices

Dell's latest update to the Compellent Storage Center includes a novel approach to tiered SSDs, but it comes with caveats

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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.

To combat these two problems, both kinds of SSDs -- SLC and MLC -- are typically equipped with more raw capacity than they advertise. This allows the device to spread out write operations over a larger number of cells, which increases overall device endurance and gives the device more cells to keep empty to absorb large write workloads. This slack capacity and the intelligence built into the SSD's controller to manage it are what really separate consumer SSDs from those used in enterprise storage devices. (They also explain the capacity differentials you'll find when you shop the two markets.)

Further, SLC and MLC devices are fundamentally different in that SLC devices store only a single bit per cell while MLC devices store two or more bits per cell. This means that SLCs use fewer transistors per cell as compared to MLCs, but more transistors to store the same amount of data. Thus, SLCs can sustain a much larger write workload (usually 25 to 30 full writes per day versus three per day for MLC), and it will absorb writes three to five times faster, but are also substantially smaller and more expensive than MLCs. However, SLC and MLC SSDs are nearly equal in terms of read performance (with MLC perhaps 2 to 3 percent slower) -- a fact that's crucial to understanding Dell's approach to flash tiering.

The Compellent's secret sauce

Even before its acquisition by Dell in 2011, the Compellent system's main claim to fame was its Data Progression (DP) tiering software. DP's job is to free up capacity in faster and more expensive tiers of storage by moving data into progressively slower and more economical tiers.

For example, suppose your Compellent array consists of a top tier of fast, expensive 15,000-rpm SAS disks and a bottom tier of much larger, slower, and less expensive 7,200-rpm NL-SAS disks. Unless you configure it not to, the array will split incoming data into pages and write them across the disks in the top tier. Because Compellent arrays implement RAID at the page level, your array can choose which RAID level to use on a per-page basis. Since writing in RAID10 is faster than RAID5 or RAID6 (given that only two write operations are required and no parity must be computed), it will use RAID10.

However, top-tier disk capacity is typically limited and fairly expensive. The array won't want to leave that storage sitting there for long unless there's a good reason to. That's where Data Progression comes into play. At some point every day (7 p.m. is the default) DP will run as a background process on the array, moving data to different tiers and changing the RAID level based on how heavily the data has been used and what policies you have set. DP will even differentiate between the faster outer rim of NL-SAS disks versus the slower inner tracks, creating a sort of tier within a tier (Dell calls this licensed feature FastTrack).

If that block of data you wrote has been written once and not read again since, it might be moved to the bottom tier and restriped using RAID5. If it had been read more frequently, it might be left in the faster, top-tier storage, but still restriped to RAID5, which is just as fast as RAID10 from a read perspective and takes up quite a bit less space. In both cases, these changes are made by a low-priority process that you'd configure to run at a time when the array isn't under peak demand.

All in all, Data Progression's job is to give you the read and write performance of the top tier of disk for the data that needs it, while allowing you to leverage the economy of lower tiers of disk for less frequently used data. In situations where the array is sized properly, DP does this exceedingly well.

Dell Compellent Storage Center write-intensive and read-intensive tiers
The Compellent Enterprise Manager will keep tabs on the usage of your two flash tiers -- the write-intensive SLC SSDs and read-intensive MLC SSDs.
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