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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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 Because it knows the snapshot data is far less likely to be read from once it has been replaced by newer data in the active volume, it will typically move those pages to a more economical tier during its next 7 p.m. run.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.

The goal of this process is threefold:

  1. To consistently offer hosts the write performance of the SLC SSDs
  2. To shield the MLC SSDs from the volume of writes that the SLC SSDs will endure
  3. To take advantage of the much greater capacity offered by the MLC SSDs to make the whole thing affordable

Thus, Dell's approach to flash tiering succeeds in leveraging the best that SLC and MLC devices bring to the table while avoiding the sweeping compromises made by single-tier deployments of "mixed-use SLC" (SLC with less wear-leveling capacity) and "eMLC" (MLC with added wear-leveling capacity). Said another way, it's much more like having a tiered 15K SAS/7.2K NL-SAS spinning-disk array that can give you the benefit of both types of media versus having a single-tier 10K SAS spinning-disk array that gives you something in between.

Yes, there's a catch

It's rare that an engineering decision doesn't have some kind of drawback. In this case, the catch is found in the creation of those snapshots that are so vital to the tiered-flash model. If data is immediately moved from the write-optimized SLC tier to the read-optimized MLC tier upon creation of a snapshot, there's an obvious cost to doing that. The load on the SLC tier will increase as data is read out of it and written into the MLC tier, and this can't help but impact performance on the SLC tier whenever host I/O is driving those SSDs to their limits. Worse yet, pages that are migrated from one tier to the next have to be locked during the operation, and this can cause contention in very high-I/O situations given that committing data to the MLC tier takes three to five times longer than reading it from the SLC tier.

To test the impact of this, I created a worst-case scenario in the lab. I set up a series of volumes and started directing a breakneck read and write load at all of them. In my case, it was a stream of randomized 4K I/Os with a 70/30 mix of reads versus writes (very roughly approximating an OLTP workload). This workload was isolated to a fairly small footprint on the array (about 80GB in total).

Dell Compellent Storage Center drive endurance
Enterprise Manager will also help you keep an eye on the health and wear of the SSDs.
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