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Review: Dell Compellent storage delivers flash speeds at disk prices

Matt Prigge | Dec. 12, 2013
Dell's latest update to the Compellent Storage Center includes a novel approach to tiered SSDs, but it comes with caveats

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.

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.

Having your cake and eating it too
Accomplishing this same feat when tiering between two tiers of SSDs is something of a different animal. Whereas Data Progression runs once a day in spinning-disk configurations, it operates continuously in tiered-flash configurations. In the case of tiered flash, DP is also heavily linked to the array's snapshotting mechanism.

Like many fully virtualized arrays, Compellent arrays implement snapshots ("replays" in Compellent parlance) at a page level. When you write data into a volume, that data is split up into pages and written to disk. If you create a snapshot, those pages and any pages written before them are marked in a database as being part of that snapshot, but effectively nothing else happens -- no data is immediately moved anywhere. Later, if some of the volume is rewritten with new data, that data is split up and written into different pages on the disk; the original pages still exist and are ready to be referenced if the snapshot is ever needed. Once a snapshot is deleted, the pages that comprised it are freed to be overwritten.

In spinning-disk configurations, Data Progression treats pages that are part of a snapshot differently than it treats active data. Because it knows the snapshot data is far less likely to be read from once it has been replaced by newer data in the active volume, it will typically move those pages to a more economical tier during its next 7 p.m. run.

However, in tiered-flash configurations, Data Progression doesn't wait for 7 p.m. to roll around to make tiering decisions. Instead, immediately upon the creation of a snapshot, Data Progression will punt data out of the top tier that is backed by expensive, write-optimized SLC SSD and write the data into inexpensive, read-optimized MLC SSDs.


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