Flash storage debate heats up over QLC SSDs vs. HDDs

QLC flash storage is exerting pressure on spinning disk for mass storage and archiving use cases -- but HDDs will stick around for years if not decades, industry experts predict.

Solid-state drives that use dense QLC NAND are becoming an intriguing alternative to spinning disk because they can reduce costs over flash options that store fewer bits per cell. Pure Storage and Vast Data -- two vendors that make arrays with quad-level cell (QLC) flash -- went so far as to unequivocally predict that QLC SSDs would replace hard disk drives (HDDs) during a panel discussion at last week's Flash Memory Summit.

But most storage experts agree that, much like age-old tape media, cheaper HDDs won't disappear any time soon -- especially with some of the world's biggest storage users still relying on them to keep costs down.

Los Alamos National Laboratory (LANL), for instance, uses HDDs to store close to 300 petabytes (PB) of cool data that would not benefit from the higher IOPS that flash drives can deliver. Gary Grider, high-performance computing division leader at LANL, said he does not see QLC SSDs replacing disk at the lab.

SSD cost differential

"There is still a 4x difference in cost per capacity," Grider said. "Disks can produce reasonable bandwidth per dollar if used in mass parallel, which we have been doing for decades. So, for applications that require only bandwidth, with no latency requirements, disk is the answer -- and will be for years, we think."

Total annual capacity shipped for HDDs, SSDs and LTO tape

Grider said LANL uses the HDDs for backup, archive, transfer and staging applications, as well as slow offline indexing of cold data. He estimated that the lab also stores about 500 PB of cold data on tape and 6 PB of recently generated data on SSDs.

"The flash is growing faster by percentage than the disk, which is growing faster than the tape," Grider said. Although LANL doesn't have much QLC flash, Grider said he expects the lab's QLC SSD numbers to change "soon and fast."

Like all NAND flash, QLC technology can speed access to data over the mechanical HDDs that account for the bulk of the world's shipped storage capacity. But to realize QLC's cost benefits, storage manufacturers must overcome a number of technical challenges, including its lower endurance, performance and throughput compared with single-, multi- and triple-level cell flash that store one, two and three bits per cell respectively.

NAND flash shipments by technology type

Addressing QLC challenges

Pure Storage, for instance, built proprietary DirectFlash modules to connect the raw QLC flash directly to its storage system via NVMe and facilitate the data placement, higher throughput and lower latency necessary for its FlashArray//C to handle performance- and capacity-oriented workloads. Shawn Rosemarin, vice president of worldwide systems engineering at Pure, said during the Flash Memory Summit panel discussion that latency would be the No. 1 reason that users replace HDDs with QLC flash. "There's no doubt about it. Spinning disk is on the declining curve where it will only exist for the lowest-value data," Rosemarin said.

Rosemarin said some vendors front end QLC with TLC flash, storage class memory or DRAM to address caching and performance issues, but they run the risk of scaling problems and destroying the cost advantage that the denser flash technology can bring.

"We had to launch a whole new architecture with FlashArray//C to optimize and run