Flash storage: Guide to enterprise all-flash storage arrays
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Compellent customers technically could create an all-flash configuration as far back as October 2009 by loading up a standard array with solid-state drives (SSDs). But with a major platform upgrade in August 2013, Compellent arrays gained such cutting-edge abilities as automated tiering of sub-LUN data across different types of SSDs. Users can tier between write-intensive single-level cell (SLC) drives and read-intensive multilevel cell (MLC) drives and tack on a layer of hard disk drives (HDDs) if they choose.
Future Compellent SC8000 releases are due to include such capabilities as inline deduplication and compression, and quality of service (QoS), as well as to add support for RESTful application programming interfaces and OpenStack cloud technology drivers. Those enhancements would bring the product into line with some of the more feature-rich all-flash arrays.
The Compellent all-flash array is among the highest-capacity options in the market, with a maximum raw capacity of 1.6 PB in a dual-controller setup, and the product stands out with its support of 16 Gbps Fibre Channel and Fibre Channel over Ethernet storage networking.
Compellent is targeted to medium-sized to large organizations, while Dell's line of iSCSI-only EqualLogic arrays typically takes aim at the use cases of small- to medium-sized businesses.
Compellent's list price is $130,000 with six SLC SSDs and six MLC SSDs, and Dell claims the Compellent all-flash arrays are priced from $5 per gigabyte to $10 per GB with 48 SLC SSDs and 240 MLC SSDs.
Like Compellent, EqualLogic launched its first all-SSD-capable model in 2009. But the December PS6210S release takes advantage of controller and firmware technology redesigned specifically with flash in mind. The end result is a performance claim by Dell of 1.2 million IOPS in a configuration with a virtual pool of eight all-flash arrays.
The PS6210S all-flash EqualLogic array supports only the high-performance but more expensive SLC flash, as the industry increasingly moves to cheaper MLC technology. Dell supplied a starting price of $8 per GB for the PS6210S, based on a configuration with two dozen 800-GB SSDs, placing it roughly in the middle of the pack of the lines of all-flash arrays.
The most aggressive all-flash array vendors claim their prices are less than $5 per GB, but those products often employ data reduction technology to hit those numbers. Neither the all-flash EqualLogic arrays nor Compellent arrays support inline deduplication and compression, two of the most important storage-saving features. They support only post-process deduplication for file data within a unified storage system via the Dell Fluid File System, and Compellent this month added support for post-process compression, according to a Dell spokesperson.
Dell's EqualLogic division has credited the PS6210S array's redesigned controller and software for allowing it to stake a claim of flash performance at the price of disk. The company declined to supply an estimated price per IOPS.
EqualLogic supports such enterprise storage features as thin provisioning, snapshots, replication and data encryption, and unlike Compellent, it doesn't charge an extra fee for any of them. All software is included with the purchase of the array.
Like most all-flash array vendors, Dell said it would replace flash drives in EqualLogic arrays if they wear out before the warranty expires, with no limitation on the number of writes. By contrast, Compellent replaces flash drives only if they are within the SSD's "rated life" -- the maximum amount of bytes that can be written to the device. Compellent's warranty covers defects in workmanship and/or materials, but it does not cover problems related to drives reaching their maximum-rated life, according to a Dell representative.
Dell claims the Compellent SC8000 has no performance degradation during controller failure in typical cases. There could be as much as 50% degradation if the controllers are running at peak performance and maximizing I/O and bandwidth, but Dell claims that is rare if a system is properly sized. Compellent uses redundant active-active controllers.
EqualLogic arrays avoid performance degradation with active-standby controllers. Only one controller is used in normal operating, while the second controller mirrors the cache of the active controller.