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Dell Compellent Storage Centre 6.4 review: MLC support brings down the cost
This major update of Dell's Compellent storage technology adds an additional tier of low-cost MLC SSD drives. Also in the mix is a new high-density HDD enclosure, plus deduplication and compression for the Fluid File system clustering option.
Brings all-flash storage arrays down to a more affordable price
3-tier data progression for enhanced performance
Wear level (endurance) monitoring
5-year warranty extended to SSDs
New SC280 enclosure raises HDD capacity to 67.2TB per rack unit
Careful sizing of SSD configuration required to gain maximum benefit from flash-optimised tiering
Dell has refreshed its Compellent enterprise storage family with a new release of the 6.4 Storage Centre software to support data tiering across different SSD technologies as well as conventional hard disk. Affordable all-flash arrays have also been introduced along with an ultra-dense HDD enclosure and an update to the Fluid File System clustering OS — all designed to further extend the capabilities of this already very scalable enterprise storage family.
To fully appreciate Dell's enhancement of the Compellent technology it acquired at the end of 2011, you have first to understand the difference between single-level cell (SLC) and multi-level cell (MLC) SSDs. Simply put, the flash memory used to build an SSD drive comprises millions of individual cells that, with SLC, can only hold one bit of data each. With MLC, however, two or more bits can be stored per cell, increasing capacity while at the same time reducing costs. This is why most consumer SSDs are based on MLC technology.
One of the downsides of MLC, however, is the need for additional processing to accommodate common multi-bit errors. This makes MLC much slower, especially when writing data. The cells in MLC flash also wear out much quicker than with SLC.
So, in summary, SLC drives are faster than MLC and last longer, but are limited in size and are vastly more expensive. MLC drives, on the other hand, are more capacious and more affordable.
Making a difference
Having got your head around the technology you also need to know that in previous Compellent products, the only SSD option was to include expensive SLC drives alongside traditional spinning HDDs. Given this arrangement the automated data progression technology in the Compellent software could be configured to migrate frequently-used data to SSD for maximum performance and little-used data to disk for reduced cost.
The end result was improved throughput, but at a price. Moreover, because of the cost of SLC, an all-flash array was deemed too expensive for most applications.
Now, however, you can mix together expensive SLC drives and cheaper, larger MLC SSDs to create a much more affordable all-flash array. Plus you can take advantage of enhanced 3-tier data progression in the 6.4 software release to migrate across both SSD tiers and, optionally, onto hard disk.
This simple change enables Dell to claim 'flash for the price of disk' — at least compared to a conventional array of 15,000rpm enterprise HDDs. Moreover, you still get benefit of SLC performance where it matters most: for write-intensive operations, with a further boost to read-intensive throughput compared to hybrid SSD/magnetic disk arrays.
No major changes have been made to the Compellent hardware to accommodate the new MLC SSDs or the new 3-tier data progression capabilities, other than to introduce new flash-optimised configurations. Based on twin Compellent SC8000 controllers, these can be specified with a mix of 400GB SLC SSDs and 1.6TB MLC drives in batches of six; Dell is currently using SANdisk drives to deliver these options.
The system we tested had a pair of redundant SC8000 controllers plus an SC220 enclosure equipped with six write-intensive 400GB SLC SSDs for Tier 1 data, plus six of the read-intensive 1.6TB MLC SSDs for Tier 2. Priced at £155,000 (ex. VAT), this is still an entry-level configuration — but one that consistently delivered 200,000 IOPS in read-intensive tests run using the Vdbench workload generator.
That figure fell to around 100,000 IOPS with a mixed workload, but it's still impressive, supporting Dell's claims to deliver fast all-flash storage at around a fifth of the cost of alternative SLC-only products.
For companies wanting more, the remaining slots in the 24-bay SC220 can be filled with additional 6-packs of SLC/MLC SSDs to further extend an all-flash array, or a combination of SSDs and 1TB SAS drives for a hybrid solution.
Confusingly the company has also added a new SC280 enclosure to the Compellent hardware line-up, but this has nothing to do with SSD. Rather it lets you configure up to 84 conventional 4TB 3.5-inch SAS hard disks, making it possible to support up to 336TB in just 5U of rack space. Or 67.2TB per rack unit, if you prefer.
The Compellent management software also comes in for a few tweaks in version 6.4 with the addition of flash-optimised profiles to support the SLC/MLC mix. Some care is needed when configuring an all-flash array, however, as data progression between tiers also becomes a continuous process rather than being confined to off-peak periods of low demand. In theory, this could have an impact on performance, especially at periods of high load. Fortunately this can be avoided by sizing the SLC tier to ensure it has the capacity to handle write-intensive workloads with the option of also employing profiles to prevent automatic data progression being applied to heavily used volumes.
Another plus is the ability to monitor SSD wear levels (referred to as endurance) from the Storage Centre console which, when added to the use of MLC mainly for read-intensive operations, enables Dell to extend the 5-year warranty carried by its magnetic disks to the SSDs in Compellent arrays.