Western Digital and SanDisk jointly announced a new partnership earlier this week. The two companies are going to work together to develop their own brand of solid state hybrid drive (SSHD), claiming that the combined WD Black will debut in 5mm, 7mm, and 9.5mm form factors, offer 500GB of storage, and compete against Seagate’s own hybrid drives like the Laptop Thin lineup. Unlike Seagate, which is on its third iteration of hybrid technology, Western Digital is coming at this fresh – and it’s not using the same cache approach as its competitor.
To date, we’ve seen two general methods of data caching come to market. First, there’s the approach Seagate takes with the Momentus XT/Laptop Thin series. The entire cache process happens below the OS level; there are no drivers, no configuration details, no visible sign that any data is being cached at all. Up until recently, Seagate didn’t cache writes, which limited the potential for data loss.
The Laptop Thin series introduced some support for write caching (in which data is reported as having been written to the drive before the actual write is finished). To ensure that data wasn’t lost, Seagate added capacitors to the final product. In the event of an unexpected power loss, the Laptop Thin series stores enough power to finish writing data before powering down. Since we’re talking about NAND, that doesn’t take long.
The other method of caching uses software like Smart Response Technology or Nvelo’s Dataplex. Nvelo has been acquired by Samsung since we wrote our SSD cache wars article, but the approach hasn’t changed. Both Intel and Nvelo install an extremely low-level driver that monitors file system activity and decides which data should be cached on the SSD to ensure maximum performance. Both drives are hooked to the system south bridge, which handles the synchronisation. Data loss is possible in these scenarios, though we never encountered a problem with RST or Dataplex, and both companies stressed that they take steps to minimise the chance of data being lost.
So, how does WD cache its data? By using a mixture of both approaches. As Tech Report explains, WD has gone for the driver-level approach for file management, but the NAND flash and magnetic media don’t communicate over the main system SATA bus; they’re directly linked by an internal SATA bridge chip. Both reads and writes will be cached, but Western Digital could theoretically incorporate a capacitor approach like Seagate’s to ensure that write data is saved in the event of a power loss.
Western Digital is incorporating 8-24GB of NAND in its drives (Seagate offers a static 8GB of flash) and has heavily customised the firmware. PC manufacturers will apparently have the option of choosing between Western Digital’s own driver or an Intel driver, with TR reporting that the WD driver was built in-house and is capable of using system memory as part of the caching scheme. Presumably the WD driver also supports AMD systems; the Intel driver would obviously be limited to Intel’s own chipsets.
For now, this is a mobile-only game, but it’s easy to see where the road will lead if these hybrid products take off. The VelociRaptor family is an easy target for this type of integration, and as we’ve seen in previous testing, combining the VR with a cache solution does improve overall performance in some tests.
This is the logical next step for the cache drive approach. Integrating both drives into a single package allows the same performance profile in an Ultrabook where multiple drive bays aren’t an option. Western Digital isn’t shipping samples quite yet, but we’ll have performance figures as soon as they’re available, so stay tuned.
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