SanDisk Z400s SSD Review

Specifications, Pricing, Warranty And Accessories

Just a few weeks ago, we saw SanDisk's Z400s on sale in an almost unbelievable bundle. For $128, you could get four 128GB drives, dividing out to 25 cents per gigabyte of flash. A friend of ours took the bait and purchased a pack. If the drives turn out to be duds, he at least has a good story and some nice-looking coasters.

Prior to 2008, if you were a power user, your PC had a Raptor inside. Serious enthusiasts owned several in RAID 0 or 5. Western Digital helped make storage cool for the first time. The company even had one model with a clear section on top so you could see the mechanical parts moving.

The Raptor was the first and only consumer 10,000 RPM hard drive. It represented a radical shift from other storage devices of the day (most computers shipped with 5400 RPM spindles). I/Os were naturally slow and something had to be done to feed the dual-core CPUs that started shipped a few years before. The Raptor helped teach us that high IOPS, and not clock rates, made PCs more responsive.

In 2008, JMicron released its JMF602 controller. Suddenly, companies specializing in DRAM were able to take on established hard drive makers. Storage reviewers, marketing materials and consumers mainly measured disk performance by throughput. The large numbers were easily understandable on retail boxes. The top-tier VelociRaptor 300GB kicked out 115 MB/s along the outer edge of its platters, but dropped to half that on the slowest part. In comparison, JMicron's JMF602 DRAM-less controller could read at 135 MB/s and didn't rely on moving parts subject to mechanical limitations.

The problem with early DRAM-less SSDs was that they were optimized for high sequential performance. Operating system I/Os move very random, very small bits of data. Without DRAM, the JMF602 was ill-equipped to handle that workload, and it didn't take long for owners to figure it out. The early SSD stuttering issues are infamous today. JMicron's next controller added DRAM to cache both incoming data and increase the table map speed. DRAM was a savior, but it also increased manufacturing costs.

Over time, fresh innovation allowed controller vendors to work around DRAM. A small start-up from Silicon Valley called SandForce broke new ground with DRAM-less SSD technology. The lessons learned gave other controller designers hope. But outside of SandForce, few designs have succeeded.

SSDs are common in enthusiast PCs. Now the problem is that we already have them, so what's going to make us want another one? Appealing to a wider audience requires SSD prices to drop. Ordinary and expected flash price reductions are not enough to get us there. If we want to push solid-state storage into lower-end systems, the expensive DRAM has to go.

A number of companies have worked on DRAM-less controllers for OEM adaption. Today we look at one of the first designed specifically for OEM customers ready to use SSDs as a marketing check-box in low-cost PCs.

Technical Specifications

The Z series, depicted above, serves the embedded market for products like digital signage and entry-level PCs currently populated by low-cost mechanical hard drives. Moving these machines over to SSDs makes them last longer, since they continue to perform well.

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Since the Z400s serves two roles, SanDisk released it in multiple capacities. In this review, we're only focusing on the 128GB model, though we'll reference the 256GB drive as well. Our 128GB sample was purchased for evaluation. SanDisk never intended to have its entry-level Z400s reviewed. But this model interests us for a couple of reasons. First, it'll eventually ship in low-cost notebooks from major OEMs (once they validate it). Four- and five-hundred-dollar notebooks with 128GB SSDs employ this type of drive. And they'll also show up in the channel at low price points. Of course, that's bound to get our attention. 

The 256GB model doesn't ship in mSATA trim. Other than that, each capacity point is available in mSATA, M.2 and 2.5-inch form factors. For the most part, sequential read performance is equal across the board, except for the very small 32GB model. Sequential writes, on the other hand, vary quite a bit, from 342 MB/s all the way down to 48. As you might expect, the 32GB Z400s drives are intended for embedded applications with low write performance requirements.

At the core of SanDisk's Z400s is the new Silicon Motion SM2246XT DRAM-less controller. At this year's Computex, we also saw competing solutions from JMicron, Marvell, Seagate and Phison. The Z400s with Silicon Motion's SM2246XT is already being shipped, while the other companies are either working on design wins or are still in pre-sales stages.   

Estimates from knowledgeable product managers claim that DRAM makes up 10 percent or more of an SSD's overall cost. The new crop of low-cost SSDs is not meant to compete with high- performance drives. Rather, they target hard disks that currently ship in entry-level systems. By enthusiast standards, inexpensive DRAM-less drives are very slow. But when the scope shifts to hard drives, they look a lot more appealing.

Pricing, Warranty And Accessories

Our Z400s shipped in a pack of four 128GB drives that sold for just $128. The special price expired before we finished writing this review. But at the time of writing, you could still snag the Z400s for $45 on Newegg Business. The 256GB model is a better value at $80. Still, as of right now, the Z400s isn't price- or performance-competitive in the channel. Plextor recently released its M6V with MLC flash and DRAM. Adata introduced its SP550 with TLC flash and Silicon Motion's SM2256 controller. At the 256GB capacity point, both drives outperform the Z400s and cost the same or slightly more.

SanDisk does cover the Z series with a three-year warranty limited by the amount of data written to the drive. The 128 and 256GB models are rated at 72 TBW.

The Z400s is supported by SanDisk's Dashboard management software, which also facilitates access to Apricorn's cloning utility.