Self-Healing Storage Systems Boost RAID Capabilities: Page 2 of 2

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Atrato is pitching the Velocity 1000 to video-on-demand, IPTV, and video-surveillance markets, where storage arrays may be installed in branch security offices, cable system distribution sites, and other locations that don't have professional storage administrators to swap drives and may not even be easily accessible to a dispatched service technician.

The Velocity 1000 is based on the SAID module. Each SAID contains 160 2.5-inch laptop-style 360-GB drives, providing 47.5 TB of usable space, taking into account both the overhead of parity information for RAID-5 and the allocation of spare space. The drives are mounted in counter-rotating pairs arranged so air can flow over all sides.

At $2 per gigabyte, the Velocity 1000 is priced competitively with low-end serial ATA arrays but offers many times their performance. A Velocity 1000 system has one or more controllers, with 4-Gbps Fibre Channel access ports and SAID cabinets. The controller is connected to the SAIDs via multiple SAS connections. A basic one-controller/one-SAID system costs about $100,000, but video-on-demand providers won't want just one. Getting this kind of capacity and performance from a conventional array would cost several times as much.

The Velocity 1000 supports RAID-1, RAID-5, and triple mirror virtual volumes. A low duty cycle option will shut down duplicate or triplicate drives in shifts to reduce the workload of each drive by as much as 60%. The Velocity 1000 also performs background disk scrubbing and uses the diagnostic data it collects to duplicate the data from the drives most likely to fail into the spare space on other drives to reduce the amount of data that needs to be rewritten during a rebuild.

Xiotech acquired ISE, the technology in its Emprise 5000, from Seagate Technology. The ISE is a 3U module that mounts 20 3.5-inch or 40 2.5-inch drives on two aluminum frames, called DataPacs, in back-to-back pairs, so the drives' rotational vibrations cancel out and air can flow over the entire system. Each ISE also includes dual active/active controllers with 1 GB of battery-backed-up cache and 4-Gbps Fibre Channel interfaces.

The basic building block of an Emprise storage infrastructure is the 1.1-, 2.4-, or 8-TB ISE. As you add capacity, you're also adding more RAID controllers and more cache memory. On a more conventional array, in contrast, adding more drive shelves for new applications would mean sharing I/O channels and cache.

Xiotech's Emprise 7000 adds a controller running the same Xiotech ICON Manager GUI as its Magnitude 3D array line to two or more Emprise 5000 ISEs and can share storage with the Magnitude 3D in virtual volumes while adding replication to the feature set. An Emprise 7000 system can manage as many as 64 ISEs for as much as a petabyte of storage.

Both systems post some sizzling performance: Atrato says a single SAID Velocity 1000 can support 3,600 simultaneous video streams or 11,000 I/Os per second. The Emprise 5000 is the current record holder in price/performance on the SPC-1 and SPC-2 benchmarks: A single ISE churned out 5,892 I/Os per second for $20,800, for a net price/performance figure of $3.52 per I/Os per second, less than 25% of the cost of conventional arrays.

Critics have pointed out that the failure avoidance and self-healing techniques used in these arrays can result in performance variations as drives are throttled back and/or disabled. This results in fewer positioners being active to serve data. However, given the high performance these systems provide, we don't see small variations in performance as an issue for the vast majority of users. And with multiterabyte drives on the horizon, IT departments need to look at RAID with fresh eyes.