loading
loading
FLASH A look at DapuStor’s combined SLC and QLC SSD China’s DapuStor has developed a dual-mode J5060 SSD, proving a mix of QLC capacity and pseudo-SLC (pSLC) speed. It is as if the drive has a configurable pSLC cache. How does it work and what are the benefits and trade-offs? First of all, an SLC drive is designed so that the cells in its NAND merely have to register two voltage states, effectively on or off. That means the voltage widow, the size of the voltage gap between the two states is wide, state sensing is simpler, read and write speeds are faster, error rates are consequently lower, and endurance, the number of read-write cycles it can endure, is at the 50,000 to100,000 level. A QLC drive, with its four bit state levels, has 16 separate voltage states which have to be sensed. The voltage window per state is much narrower, error rates are increased, read and write speeds are lower. Cell endurance is lower; we’re looking in the 1,000 write cycle area. A pSLC section of the drive layers SLC characteristics on top of this, but the cells and process are still those of a QLC part, not an SLC part. The sensing margins, program algorithms and long-term wear behavior are not the same as for a real SLC drive. They approach that of the SLC drives but don’t reach it. DapuStor says it has altered the dual-mode drive’s firmware so that: - Die-level isolation minimizes interference from QLC workloads. - Optimized SLC reserve allocation reduces write amplification, improving endurance and steady-state performance. - Region-aware I/O scheduling prioritizes latency-sensitive data across the entire data path. According to DapuStor, each dual-mode SSD delivers more than 7× the random-write IOPS of a QLC-only SSD. It says that, in 4 KB random-write tests, the pSLC region achieved an average write latency below 8μs, making it well suited to database journals, write-ahead logs (WALs), and metadata caches. Kioxia’s real SLC FL6 SSD, using PCIe gen 4 like the RS6050, has an 8μs write latency. Micron’s similar XTR has a 15μs write latency. Endurance over QLC-only drives is lifted, as the pSLC region supports more than 25× the write cycles of QLC operation. There are some other advantages to having a set of pSLC/QLC drives rather than the same number of drives divided between SLC drives and QLC drives. Assume write traffic goes to the SLC drives or pSLC regions in dial-mode drives. Envisage a chassis containing 24 SSDs. The dual-mode pSLC/QLC configuration has 24 such drives, each of which can be configured with a set amount of pSLC. A 30.72 TB J5060 could have, for example 400 GB (8%), 800 GB (14%) or 1.2 TB (20%) of pSLC with the rest being QLC. Assume we have the same config across the 24 drives, and compare that to a configuration with 23 x 30.72 TB QLC drives and 19.2 TB SLC drive. DapuStor says that, with the single SLC-23 QLC set up, the single SLC SSD handles write traffic for multiple QLC SSDs, and bursty workloads such as AI training can saturate its PCIe link while other links remain under-utilized, limiting aggregate throughput. Switch to the dual-mode pSLC/QLC config and write traffic can be distributed across all 24 drives. DapuStor says there’s a failure domain reduction as well. If, in the single SLC/multiple QLC configuration the SLC drive fails, then recovering its contents from the QLC drives “can take days and degrade cluster performance.” Using a dual-mode drive set-up a single drive failure only affects its pSLC region, greatly reducing the rebuild volume, shortening the recovery time. All-in-all considering a dual-mode pSLC-QLC drive set up could have some benefits that are absent from single mode drive usage. DapuStor says its dual-mode drives enable high capacity, high bandwidth, and low latency to coexist in a single storage system. And this is suited to AI workloads with its high capacity, high bandwidth and low latency needs. But, the obvious but, you don't get true SLC speed or endurance. It's trade-off time.
document fetched 2026-09-22 02:53 · canonical https://blocksandfiles.com/flash/2026/09/21/a-look-at-dapustors-combined-slc-and-qlc-ssd/5297708