The Toshiba conglomerate isn't having its brightest days ever. An accounting scandal, massive layoffs, and most importantly, Westinghouse's recent Chapter 11 bankruptcy filing have left the company's finances in an unenviable state. In response, Toshiba made preparations to sell off its healthy and profitable memory endeavors by spinning them off into a separate entity known as Toshiba Memory Corporation. Several interested parties have entered bids for TMC, and all might have been well for Toshiba, but Western Digital (who purchased SanDisk last year) felt that the terms of SanDisk's joint venture with Toshiba gave it some right to intervene in the sale. Negotiations have since broken down, and a whole lot of arbitration and litigation have ensued. The situation is still unfolding, but the long and short of it is that TMC remains unsold for the time being.
Even with those dark clouds over its corporate umbrella, Toshiba's memory business itself is booming. Unperturbed by the corporate-level bickering, the Toshiba-Sandisk partnership has iterated on its "Bit Cost Scalable" 3D NAND technology to reach several key milestones. In the last few weeks alone, Toshiba has announced the following BiCS technology leaps: quadruple-bit cells, 96-layer dies, and through-silicon vias. The most important thing about BiCS, however, isn't any of the above. That honor goes to the fact that BiCS NAND is finally shipping in a client SSD. Say hello to Toshiba's XG5.
|Capacity||Max sequential (MB/s)|
This review will be a bit different from our usual fare, since the XG5 isn't a retail drive. Toshiba's XG line is sold to OEMs and system integrators rather than directly to consumers. Regardless of its target audience, the XG5 is a PCIe 3.0 x4 NVMe drive with Toshiba's 64-layer BiCS NAND and a Toshiba controller running the show. For now, the drive is being produced in 256GB, 512GB, and 1TB flavors, but Toshiba tells me nothing is stopping it from making a whopping 2TB version if one of its partners asks for it.
That story seems to check out, since the sample unit the company sent me squeezes its terabyte of capacity into just two packages on a single-sided PCB. 64-layer BiCS TLC dies come in both 256Gb and 512Gb densities. Toshiba wouldn't confirm which XG5 versions use which dies inside their memory packages, but it's a safe bet that our 1TB sample uses 512Gb dies. It would follow, then, that each package should have eight dies stacked inside, but we couldn't get confirmation from Toshiba on implementation details. Specifics of the controller and its firmware will have to remain a mystery, as well. The chip bears the same "TC58" prefix we saw on the OCZ RD400's controller, but the firmware of that part has almost certainly been updated since then. We do know that it supports pseudo-SLC caching for burst writes that the DRAM cache can't handle on its own, but that's about it.
Toshiba hasn't disseminated much technical detail about how BiCS flash actually works, but we do know it's based on charge-trapping insulators rather than traditional floating gates. Although Toshiba's implementation likely differs, readers can refer back to our primer on Samsung's charge-trap-based V-NAND technology for some basic principles.
Our sample unit doesn't feature any encryption capabilities, but Toshiba does produce a variation of the XG5 which offers hardware-based full disk encryption through the TCG Opal 2.01 standard if OEM clients should want those features in their products. The company didn't reveal an endurance spec for the XG5, but it did say to expect the same endurance from BiCS TLC drives that we've seen from Toshiba's planar 15-nm MLC drives of equal capacity.
Ordinarily we'd talk price and warranty here, but neither of those things are relevant to the XG5. You can't just buy the drive off Newegg, since Toshiba's only selling it to OEMs and system integrators. Similarly, issues experienced by the end user would be addressed by the brand selling the system, not directly by Toshiba. You might ask why we're reviewing the drive, then, and here's why: some form of the XG5 will almost certainly reach retail channels. The XG3 was essentially the same drive as the OCZ RD400. Toshiba tells us a retail equivalent of the XG5 is already in the works, and there's no small chance that will hit shelves as the OCZ RD500.
Now, on to our results. BiCS flash spent a long time in the oven, and it's finally time for us to see what it can do.
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