QLC Goes To 8TB: Samsung 870 QVO and Sabrent Rocket Q 8TB SSDs Reviewedby Billy Tallis on December 4, 2020 8:00 AM EST
This test starts with a freshly-erased drive and fills it with 128kB sequential writes at queue depth 32, recording the write speed for each 1GB segment. This test is not representative of any ordinary client/consumer usage pattern, but it does allow us to observe transitions in the drive's behavior as it fills up. This can allow us to estimate the size of any SLC write cache, and get a sense for how much performance remains on the rare occasions where real-world usage keeps writing data after filling the cache.
The Sabrent Rocket Q takes the strategy of providing the largest practical SLC cache size, which in this case is a whopping 2TB. The Samsung 870 QVO takes the opposite (and less common for QLC drives) approach of limiting the SLC cache to just 78GB, the same as on the 2TB and 4TB models.
|Average Throughput for last 16 GB||Overall Average Throughput|
Both drives maintain fairly steady write performance after their caches run out, but the Sabrent Rocket Q's post-cache write speed is twice as high. The post-cache write speed of the Rocket Q is still a bit slower than a TLC SATA drive, and is just a fraction of what's typical for TLC NVMe SSDs.
On paper, Samsung's 92L QLC is capable of a program throughput of 18MB/s per die, and the 8TB 870 QVO has 64 of those dies, for an aggregate theoretical write throughput of over 1GB/s. SLC caching can account for some of the performance loss, but the lack of performance scaling beyond the 2TB model is a controller limitation rather than a NAND limitation. The Rocket Q is affected by a similar limitation, but also benefits from QLC NAND with a considerably higher program throughput of 30MB/s per die.
Working Set Size
Most mainstream SSDs have enough DRAM to store the entire mapping table that translates logical block addresses into physical flash memory addresses. DRAMless drives only have small buffers to cache a portion of this mapping information. Some NVMe SSDs support the Host Memory Buffer feature and can borrow a piece of the host system's DRAM for this cache rather needing lots of on-controller memory.
When accessing a logical block whose mapping is not cached, the drive needs to read the mapping from the full table stored on the flash memory before it can read the user data stored at that logical block. This adds extra latency to read operations and in the worst case may double random read latency.
We can see the effects of the size of any mapping buffer by performing random reads from different sized portions of the drive. When performing random reads from a small slice of the drive, we expect the mappings to all fit in the cache, and when performing random reads from the entire drive, we expect mostly cache misses.
When performing this test on mainstream drives with a full-sized DRAM cache, we expect performance to be generally constant regardless of the working set size, or for performance to drop only slightly as the working set size increases.
The Sabrent Rocket Q's random read performance is unusually unsteady at small working set sizes, but levels out at a bit over 8k IOPS for working set sizes of at least 16GB. Reads scattered across the entire drive do show a substantial drop in performance, due to the limited size of the DRAM buffer on this drive.
The Samsung drive has the full 8GB of DRAM and can keep the entire drive's address mapping mapping table in RAM, so its random read performance does not vary with working set size. However, it's clearly slower than the smaller capacities of the 870 QVO; there's some extra overhead in connecting this much flash to a 4-channel controller.
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Beaver M. - Saturday, December 5, 2020 - linkNot really. You can get 4TB TLC NVMe drives for around the same price as this QLC one.
QLC would need to be 50% cheaper at least to make any sense.
Spunjji - Monday, December 7, 2020 - linkFrom an actual manufacturing perspective, even 33% cheaper would be a reach. We'll be lucky to see 25%.
Beaver M. - Tuesday, December 8, 2020 - linkI agree.
Thats why I think they are trying to fool buyers with QLC.
DeathArrow - Saturday, December 5, 2020 - linkWhy is Anandtech the last to conduct reviews on many products? By the time you review the latest graphic cards or SSDs, there is not much interest since most people already get their info from other outlets.
Beaver M. - Thursday, December 10, 2020 - linkAnandtech has declined massively this year. Most of their "articles" are "Best This And That Buy Right Now" and other ads nowadays.
Im about to remove them from my bookmarks...
Luuta - Saturday, December 5, 2020 - linkThe product comparisons would be so well and good if companies like ADATA haven't swapped out premium components from initial drive launches for far cheaper ones, with dramatic loss in performance. It's fraudulent. It also makes a nonsense of all these reviews and the comparisons because the consumer won't see any of it, once the first batch is off the production line. These companies need to be held accountable by law to stop them ripping off consumers with their own counterfeit products. Until then, I no longer believe any bench marking of either SSD or hard drive products from any manufacturer.
Deicidium369 - Saturday, December 5, 2020 - linkHere's a tip - Stay as far away from the budget manufacturers like ADATA - for our datacenter SSDs it's all Intel Optane U.2 - and for my desktops - Samsung and looking into the new Phison based controllers that are 7GB/s R&W. I pretty much put Sabrent into that category with ADATA - only Sabrent part I have is a 2.5" to 3.5" drive sled.
Cliff34 - Saturday, December 5, 2020 - linkFor me, the sweet spot is 4 TB. I need a lot of space to store media on my laptop. Right now using 2 TB, I am doing alright. But I feel 4 TB will give me more confidence I won't run out of space.
Sadly, it doesn't look like the market is ready to move on. I've got my 2TB three years ago and prices wise, it hasn't changed so much.
Slash3 - Sunday, December 6, 2020 - linkI have two 2TB Crucial MX500s for general storage and they're only ten bucks cheaper than what I paid, over two years ago.
MDD1963 - Monday, December 7, 2020 - linkHmmm...wonder what the "Q" in QVO stands for? :)