Internals, Spectrum Controller, Testing Methodology and System Setup




Let's spin that top one around…:

If there was any doubt that these are identical parts, that last pic should settle things. The flash is BiCS3 TLC. The controller is a proprietary in-house 'Spectrum' part designed by WD/SanDisk. Note the arrangement here. The controller is centered, which WD claims helps spread heat away from the controller without the need for special copper-layered labels or heat sinks. Let's dive a bit deeper into this new controller:

Spectrum Controller

SSD controllers need to do things differently if there is any hope of beating the hefty competition out there, and Western Digital’s new Spectrum controller employs a range of unique functionality towards this end.

First, we have a restructuring of the portions of the controller responsible for handling repetitive tasks that are relatively simple on their own. These tasks hang (logically) off of each end of the controller, where it speaks to the host via NVMe or to the NAND. These pathways are handled by Sequencers on the Spectrum controller, and the way WD had sped these processes up can be summed up with a very simple term: ASIC.

Since the NVMe and NAND architectures employed change very infrequently (years), WD can implement those protocols purely in hardware. The downside here is that you lose the ability to firmware update those portions of the SSD, but so long as they get things right the first time, there should be nothing to worry about here.

Another aspect of the Spectrum controller that enjoys ASIC acceleration is the ‘multi-gear’ error correction. As with modern HDDs, NAND always has some degree of error correction at play, and WD has implemented multiple stages of LDPC error correction in hardware. They are all reasonably quick but use increased power as the complexity of the error correction increases. If a given error is too large, it ‘falls through’ the LDPC stages and now we have the CPU / firmware kick in and attempt some additional DSP tricks (threshold adjustment, etc.) to get the data read successfully. A last-ditch effort available is a ‘RAID-like’ recovery where some XOR data may be available for this page(s). It’s not a full RAID-5 of flash memory dies, but anything to help improve your chances here is a good thing.

Before we move on, let’s touch on caching. The bulk media on these new parts is TLC, but there is an SLC cache. nCache 3.0 is WD’s label for the caching scheme here, and it functions similarly to other high-end SLC caching SSDs. Incoming writes go to the cache whenever possible, and that cache is emptied (folded) into the TLC space during any idle periods. Should the SSD see enough sustained writes that the cache becomes saturated, it shifts to ‘direct to die’ TLC writes, which are slower, but still faster than the case where the SLC cache was being simultaneously filled and emptied. In these cases, it’s more efficient to just let the earlier data sit idle in the cache area until the workload subsides and the cache can be emptied during the next idle period.

Testing Methodology

Our tests are a mix of synthetic and real-world benchmarks. IOMeter, HDTach, HDTune, Yapt and our custom File Copy test round out the selection to cover just about all bases. We have developed a custom test suite as off-the-shelf tests just no longer cut it for in-depth storage testing. More details on the next page. If you have any questions about our tests just drop into the Storage Forum and we'll help you out!

Test System Setup

We have several storage testbeds. A newer ASUS P8Z77-V Pro/Thunderbolt and a Gigabyte Z170X SOC Force (for RAID testing). Future PCIe and SATA device testing, including this review, take place on an ASUS Sabertooth X99, which comes equipped with USB 3.1, M.2, and can also handle SFF-8639 (U.2) devices with the proper adapter.

PC Perspective would like to thank Intel, ASUS, Gigabyte, Corsair, Kingston, and EVGA for supplying some of the components of our test rigs. 

Hard Drive Test System Setup
CPU Intel Core i7 5820K @ 4.125 GHz
Motherboard ASUS Sabertooth X99
Memory 16GB Micron DDR4 @ 3333
Hard Drive G.Skill 32GB SLC SSD
Sound Card N/A
Video Card EVGA GeForce GTX 750
Video Drivers GeForce Game Ready Driver 347.88
Power Supply Corsair CMPSU-650TX
DirectX Version N/A
Operating System Windows 8.1 Pro X64 (update)
  • PCPer File Copy Test
  • HDTach
  • HDTune
  • IOMeter
  • YAPT
  • PCPer Custom SSD TEST SUITE!!!
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