RAM cache makes hybrid hard drive fly

M-Cell boosts slow disk I/O.

A new hard drive from Japanese company DTS has added 1GB of DRAM cache to a 2.5-inch serial ATA disk.

The performance ramp from the M-Cell Hyper-Drive speeds up disk I/O, it is claimed, by between ten and 100 times, providing near solid-state disk performance at an affordable price.

M- or Memory-Cell is a 3.5-inch enclosure containing a 2.5-inch, 5400rpm, 120GB SATA drive, 1GB of write-through DDR2 DRAM cache, a CPU chip, a real-time operating system, plus the associated electronics.

It is claimed that data access times are faster than those seen on a 3.5-inch, 7,200rpm drive. DTS said that users should throughput of 110MB/sec in random read tests for data sizes of 64KB to 512MB. When the data being accessed is larger than the cache then throughput will slow. It says a 3.5-inch, 7,200rpm SATA disk will only deliver 60MB/sec.

Existing hybrid drives used slower, and cheaper, NAND flash memory as a cache. Samsung has a hybrid drive line with 256MB or 512MB of flash memory and disk capacities of 80, 120 or 160GB. These are intended for notebook PC use and will work with Vista's ReadyDrive feature to speed disk I/O. Samsung has prototyped a drive with 4GB of flash cache.

Seagate offers its Momentus 5400 PSD hybrid drive with a 5,400rpm 2.5-inch SATA drive and 256GB of flash memory. It has a SATA 1 interface operating at a maximum of 150MB/sec. This speed will not be seen in real-life conditions.

Hybrid drives are becoming popular because they provide disk-based capacity with flash-based performance. The MCell DRAM cache-based drive will provide even more of a performance boost.

DTS also supplies C4 application acceleration software. Its MCell technology has previously been used as a cache for RAID arrays. Now it is being used for single drives.

DRAM is volatile, meaning contents are lost if power is switched off or fails, unlike nonvolatile flash memory. However, according to eStorage: "There are built-in capacitors and an intelligent UPS to ensure data in memory is written to nonvolatile disk in the case of power failure."

M-Cell is available in 80GB, 120GB and 160GB capacities. The 80GB product costs about US$136.

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Chris Mellor

Techworld.com
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