Hybrid NAND, too
Unlike the last two generations of Momentus XT hybrids, which use SLC NAND for their flash caches, the Laptop Thin SSHD is equipped with MLC memory—sort of. According to Samsung's decoder ring, the NAND on the drive's circuit board is indeed multi-level cell memory. However, the Laptop Thin employs a new flash subsystem with a special "combo mode" that treats part of the flash as SLC and the rest as MLC. Seagate hasn't answered our questions about exactly how this combo mode works, but there doesn't appear to be any customization at the flash level. I suspect Seagate has simply elected to write only one bit per cell for the portion of NAND allocated as SLC memory and two bits per cell for the rest.
To understand the benefits of this approach, it helps to know how bits are actually stored. Writing to flash memory is achieved by causing electrons to migrate into the cell, generating a negative charge that changes the cell's threshold voltage. After each write, a control voltage is applied to read the cell and verify its contents. If the control voltage is higher than the threshold, current flows through the cell. If it's not, the process is repeated with a higher control voltage.
With one bit per cell, SLC memory only needs to worry about whether the cell's threshold voltage represents a 0 or 1. The extra bit in MLC NAND allows for values of 00, 01, 10, or 11, which means more control voltages need to be applied to read the data in the cell (or to verify a successful write). Cycling through those additional voltages takes time, and SLC flash tends to have faster write performance than MLC as a result. SLC memory also offers better endurance than the MLC stuff. Over time, as flash cells are written, electrons slowly build up in the insulator layer, shrinking the voltage range that can be used for programming. SLC has to differentiate between fewer values within that limited range, making it more tolerant of normal flash wear.
Seagate doesn't publish an endurance specification for the Laptop Thin SSHD's flash component, but the drive is covered by a three-year warranty. Even for worst-case workloads, Burks says there's a "really high level of wear-level margin."
Burks also told us that the hybrid NAND in the Laptop Thin SSHD is faster than the older SLC memory inside the Momentus XT, although he didn't have specific numbers. The Momentus can read data from its flash cache at up to 180MB/s, so the Laptop Thin is faster than that. If 180MB/s seems a little sluggish, keep in mind that the SSDs pushing into 500MB/s territory are typically 240-256GB models with as many as 32 individual NAND dies. The Laptop Thin SSHD's 8GB flash component has only two dies, giving it much less parallelism to exploit. (You can read about the effect of SSD capacity on performance in this article.)
The write speed of the last-gen Momentus XT's flash was quoted as 100MB/s, but that was for transfers coming from the mechanical platters. Interestingly, Seagate claims the Laptop Thin's "average data throughput" rate is also 100MB/s. Due to its combination of a higher areal density and a slower 5,400-RPM spindle speed, the new model's mechanical component may be no faster than the old one's.
The effective size of the NAND inside the Laptop Thin SSHD may be smaller, as well. We're talking about an 8GB MLC chip. If some of the NAND is configured as SLC memory, which has half the data density of MLC NAND, the total capacity of the chip must decrease.
We don't know how much of the NAND is treated as SLC memory, but Burks did tell us this portion of the flash is dedicated to two purposes. In addition to hosting the write cache, the SLC slice houses data associated with the Windows boot process. This section of the flash is populated automatically and reserved exclusively for boot data. Adaptive Memory's other caching activities won't encroach on its territory.
We'll test boot performance in a moment. First, I should take a moment to explain where the Laptop Thin SSHD fits into Seagate's next-generation hybrid lineup. The 7-mm Thin drive is accompanied by a 9.5-mm model dubbed the Laptop SSHD. This standard-sized mobile unit has dual 500GB platters, doubling the storage capacity of its skinny counterpart. The spindle speed is the same, and so is the caching component. Performance should be comparable as a result.
A mobile SSHD with more flash is in the works, but that variant won't come out until the middle of the year. Instead of relying on Adaptive Memory to manage the cache, this model will work in conjunction with Intel's Smart Response Technology. An SRT-compatible platform will be required, of course.
Seagate is also prepping SSHDs for desktop systems. The first 3.5" models will offer 1TB and 2TB of storage, respectively, and they'll sport 8GB flash caches like the mobile drives. Those first desktop models will sport faster 7,200-RPM spindle speeds. A higher-capacity desktop hybrid is also on the way, but it will have a slower spindle speed, likely in the 5,400-RPM range.
After several years of honing its hybrid technology with the Momentus XT, Seagate is ready to spread SSHDs across multiple platforms. This could be the beginning of a hybrid revolution.
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