In a bid to make solid state drives (SSDs) as capacious as traditional hard drives, storage suppliers have looked to insert ever more bits and bytes into NAND flash.
Quad-level cell (QLC) drives are the latest development of flash storage technology. As the name suggests, the technology stores four bits per cell.
The way QLC flash and all other NAND flash stores data is essentially the same, using an electrical charge to determine whether each cell is a “0” or a “1”. There are billions of such cells on a silicon substrate and they can be used to store terabytes of information.
Originally, flash was designed to store a single bit in one cell. This was known as SLC (single-level cell) technology. But soon it was discovered that a cell could store more than one state by using a range of voltages. This is how MLC (multiple-level cell) flash came about, with each cell storing four states that recorded two bits of binary information.
TLC (triple-level cell) extends this to eight states and can store three bits of data per cell.
QLC can store four bits of data using 16 states, which means using 16 different voltage levels. This sounds great, but there are issues.
Penta-level cell (PLC) has now also appeared on the horizon, but that’s another story.
QLC benefits and characteristics
The move to QLC as a storage medium should result in lower total cost of ownership (TCO). Read-centric workloads rely on vast arrays of HDDs to deliver results; QLC drives can achieve this with fewer drives and therefore lower cost.
This is because QLC stores one-third more bits per cell than its TLC predecessor, increasing storage density by up to nearly eight times as much as traditional HDDs. This should result in saving space within a datacentre.
QLC: More density, more challenges
Although increasing the capacity of cells has advantages in terms of increasing the amount of data that can be stored, there are a number of drawbacks.
Every time a cell is written to, it gets a little bit damaged – which means each cell has a finite lifetime. This is called endurance and is measured by the number of times a program/erase (P/E) cycle can be carried out on the flash memory.
NAND is programmed and erased by applying a voltage that sends electrons through an insulator. The location of those electrons (and their quantity) determine when current will flow between a source and a sink (called a voltage threshold), and that determines the data stored in that cell (the 1s and 0s). When writing and erasing NAND, it sends the electrons through the insulator and back, and that causes the insulator to wear. The exact number of these cycles in each individual cell varies by NAND design.
For SLC flash, P/E cycles are typically about 100,000. This drops to 35,000-10,000 for MLC, and 5,000 for TLC, although improvements in these figures are continuously being made by suppliers.
When it comes to QLC, P/E cycles were originally expected to be around 100, but manufacturers have managed to increase this to 1,000 P/E cycles.
When more data is stored in a cell, a single bit change entails rewriting the whole cell. This is because before you can change the cell, you need to know what value it already has.
The increased density that results from having 16 different voltage levels makes it increasingly difficult to tell the bits apart. So while QLC is 25% denser than TLC, it is also significantly slower. QLC read speeds are not too different from other types of flash, but sustained write speeds top out at 160MBps, which is slower than a traditional hard drive.
Cache the flash
If QLC is slower and breaks down more quickly than other flash, why bother at all?
To hide this problem, suppliers use caching techniques on QLC. Part of the drive is used as an SLC cache to improve write speeds. This cache can be written to at the speeds found in high-end SSDs with the drive controller flushing out data from the SLC cells to QLC cells. However, once this cache is full, there is a drop-off in speed as the QLC cells are directly written to.
QLC use cases
Although the write performance and durability of QLC is not that great compared with other flash technologies, it should not be ruled out for use in the enterprise. In fact, QLC flash offers similar read performance and endurance to TLC flash storage.
The greater wear and tear caused by having more bits packed into one cell means QLC is not best suited to write-heavy workloads. Fewer write operations makes for a longer-lasting drive.
Jason Echols, senior technical marketing manager at Micron Technology, says QLC brings the all-flash datacentre a step closer to broad adoption.
“While many workloads have already been migrated to flash, the holdouts have been read-focused applications like business intelligence and analytics, NoSQL databases and content delivery, video on demand and streaming, big data and active archives, and datacentre backup and restore,” he says.
Echols adds that QLC is increasingly displacing hard drives where reliability, read performance and lower power become important, such as big data, analytics, video streaming and object storage.
Artificial intelligence, machine learning and real-time analytics are good fits for QLC. Enterprises that carry out business intelligence on data can use the technology to provide near-real-time analysis for decision-making.
Databases such as NoSQL that contain metadata and rich content could use QLC to increase application performance. These workloads are ideally suited to QLC because they read data more than they write. Once data is recorded, it doesn’t change. QLC can enable quicker data reads and faster sorting of data.
Video on demand, media streaming and content delivery can take advantage of QLC because it can support many parallel requests and streams.
Archiving also suits the strengths of QLC because the technology can be used where long-term storage is required but unlikely to be written to very much.
Who makes QLC flash?
It was a big year for QLC in 2018. One of the key suppliers to offer QLC flash was Micron, which began shipping and selling the industry’s first QLC SSD in May2018.
This was followed up later in the year by Samsung with a 4TB QLC consumer SSD using 1TB V-NAND chips. Intel also launched a QLC SSD in 2018, based in the M.2 2280 format. And Western Digital debuted its 96-layer 3D NAND BICS4 chips featuring QLC.
Which array products offer QLC?
While several array makers are not yet public about QLC plans, some suppliers have announced their intentions:
Pure recently launched the FlashArray//C array, which combines Pure DirectFlash NVMe flash modules with support for QLC NAND SSDs. These are available in 1.3PB, 3.2PB and 5.2PB effective capacity.
While no official announcements have been made, some analysts believe NetApp will integrate QLC into enterprise SSDs in 2020, possibly in ONTAP and E-Series arrays.
Last year, AMAX launched its StorMax NFS storage solutions for AI and deep learning workloads. StorMax NFS features the Micron 5210 ION SSD, based on QLC technology.