The two main shortcomings of QLC NAND relative to the more mainstream TLC NAND are in write performance and write endurance. Both problems can be alleviated by the use of more NAND total, allowing writes to be spread across more NAND dies in parallel. That and the (hopes of) lower prices make QLC NAND best suited for large capacity SSDs. Thus, the 860 QVO product line starts at 1TB. Even at that capacity, the 860 QVO only needs 8 dies of QLC NAND and can only sustain writes at 80 MB/s. That means that the SLC write cache on the 860 QVO is even more important than for TLC SSDs. When working within the cache, the 860 QVO can saturate the SATA link with random or sequential writes. The cache functions much the same as the SLC cache on the 860 EVO, with a capacity that varies from a minimum of 6 GB when the drive is relatively full, up to 42 GB on the 1TB model or 78 GB on the 2TB and 4TB models. The Intel and Crucial consumer QLC drives also feature variable-size SLC caches but with much higher limits on the maximum cache size and a policy of retaining data in the cache until the drive needs the extra space. By contrast, the 860 QVO seems to take a more typical approach of aggressively flushing the cache during idle time in order to prepare for future bursts of write activity.
The performance specs for the 860 QVO when operating out of the SLC cache are typical for a mainstream SATA SSD. After the cache is full, performance drops significantly, with sequential writes showing the most severe effect. Power consumption is also comparable to Samsung’s other recent SATA SSDs, with the 1TB model requiring a little over 2W at peak and the larger models drawing just over 3W during writes. The 2TB and 4TB models have very nearly identical performance and power ratings, indicating that 2TB of QLC is sufficient to populate all the NAND channels of the MJX controller.
The warranty and endurance ratings for the 860 QVO are the other clear area where the use of QLC NAND has its impact. The 860 QVO’s warranty period is three years, typical for low-end SSDs but shorter than the 5 years that the 860 EVO and PRO carry. Write endurance is rated for 360 full drive writes, or 0.3 DWPD for the duration of the 3-year warranty. This is comparable to some of the cheaper TLC drives currently on the market, and in terms of total bytes written the 860 QVO’s rating is about 80% higher than the Intel 660p and Crucial P1, despite those NVMe QLC drives having the advantage of a five-year warranty.
The 860 QVO’s case follows the same basic design as Samsung’s other recent SATA SSDs, but is painted dark gray instead of Samsung’s traditional black. Internally, the 1TB 860 QVO illustrates how comically oversized even the 2.5″ drive form factor is compared to the requirements of modern consumer SSDs. The PCB features three main BGA packages: the DRAM, the controller, and the stack of eight 1TB QLC dies. There’s an empty pad on the back that can accommodate another NAND package. Samsung commonly packages up to sixteen NAND dies together, so even the 4 TB QVO may be able to get by with this same small PCB—placing DRAM becomes the more important problem. Samsung states that their current MJX controller supports up to 8TB SSDs, but there’s clearly very little demand for consumer SSDs in that capacity yet. The 2.5″ form factor itself can now accommodate at least 16TB, or 32TB if two PCBs are stacked in a 15mm thick drive. These capacities may show up in enterprise products, but are probably still several generations away from hitting the consumer SSD market.
Samsung has not announced a M.2 version of the 860 QVO, but that’s clearly possible if the demand is there. They can probably fit even the 4TB 860 QVO onto a single-sided 80mm M.2 card.
|Samsung 860 Family
The launch MSRPs for the 860 QVO do not compare favorably against SATA SSDs already on the market. The 860 EVO is currently below the QVO’s 15 cents per GB at 1TB and 2TB capacities, and plenty of other mainstream TLC drives are priced similarly. Samsung currently has no competition in the 4TB SATA SSD space, so their 4TB 860 EVO is substantially more expensive per GB, leaving appropriate room below for the 860 QVO. In order to be a strong competitor in the consumer market, the 860 QVO really needs to priced at no more than 13 cents per GB, and that limit is liable to come down further over the next several months as flash memory prices continue to drop and QLC yields improve.
A Note on Our Testing: The Occasional Drive Failure
Samsung provided us with samples of the 1TB and 4TB 860 QVO. Testing the 1TB model went smoothly, but the 4TB 860 QVO has run into some problems causing the drive to disappear from the system interface. These issues are most noticeable when hot-swapping the drive, which is a regular part of our SATA SSD testing routine. There have also been challenges getting the 4TB drive recognized by a motherboard during the boot process, and this causes the boot to stall indefinitely on some of our systems.
It should be noted that our testing regime is fairly streunous, and we sometimes get drive failures. It happens, and only a few of them are ever drive specific.
As a result, these problems do not appear to be specific to the 860 QVO or its use of QLC NAND and have been replicated on both the 4TB 860 EVO and 4TB 860 PRO with multiple host systems, but have not occurred with any of the smaller 860s. The 3.84 TB 860 DCT and the 4TB 850 EVO have also been trouble-free, so this isn’t a problem with 4TB SSDs in general. We are still working with Samsung to determine the scope and nature of these issues with the 4TB drives, and it is not yet clear whether there is a general compatibility problem or if our testing procedure has triggered a firmware bug that put our samples permanently into an uncooperative mood.
Because of these issues, some benchmark results for the 4TB drives have been delayed. Performance and price notwithstanding, I am unable to recommend any of the 4TB 860 series SSDs until I have a better understanding of the nature of the problems.
There are no other consumer QLC SATA SSDs on the market yet, though ADATA has announced their SU630, which will feature alarmingly low capacities. The Intel 660p and Crucial P1 are the only other consumer QLC SSDs currently available, but they are priced for the NVMe market. Most of the competition for the 860 QVO will come from SATA SSDs with TLC NAND, both entry-level models with DRAMless controllers (eg. Toshiba TR200) and more mainstream models like the 860 EVO, Crucial MX500 and WD Blue. Almost all SATA SSD product lines feature a 1TB class model, but there are still relatively few 2TB models and no 4TB competitors have been announced. This will likely change as other QLC drives come to market, and even 2TB TLC drives have been getting more common this year.
|AnandTech 2018 Consumer SSD Testbed|
|CPU||Intel Xeon E3 1240 v5|
|Motherboard||ASRock Fatal1ty E3V5 Performance Gaming/OC|
|Memory||4x 8GB G.SKILL Ripjaws DDR4-2400 CL15|
|Graphics||AMD Radeon HD 5450, 1920×[email protected]|
|Software||Windows 10 x64, version 1709|
|Linux kernel version 4.14, fio version 3.6|
|Spectre/Meltdown microcode and OS patches current as of May 2018|