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IT professionals tend to see the march of technology as monotonic and progressive -- always moving in one direction toward greater improvement, however that's measured. The impression is generally valid: No one makes CRT TVs, rotary landline phones or cassette recorders anymore because there's no market for them given vastly superior replacement technologies.
However, sometimes there's room for multiple generations of the same underlying technology because of secondary differences like performance, reliability, durability, lifespan and cost. Such is the case with NAND flash memory, where the progression from single-level cell (SLC) to multi-level cell, triple-level cell (TLC) and now quad-level cell (QLC) storage technology has left enough performance gaps between them to allow space for the different forms of NAND flash in the modern data center.
The fundamental trade-off between performance and capacity is discussed in this earlier article; here, our focus is on the two highest-density technologies, QLC vs. TLC . You might think that because QLC, with four bits per cell, is an evolutionary extension that increases flash memory density by 33% it could completely replace TLC, with three bits per cell, for high-capacity SSD uses. However, TLC technology has improved in terms of durability and performance to create roles for both types of NAND. Read on to find out more about the QLC vs. TLC debate and why you might want both in your storage systems.
The throughput vs. endurance tradeoff
Memory technology seldom gives you something for nothing; pushing the boundaries in one direction usually sacrifices features and performance in another dimension. The evolution of NAND flash memory cell technology has resulted in the tradeoff of higher density through packing more bits per cell for slower I/O throughput, higher read latency and lower endurance.
The tradeoff between throughput and endurance versus capacity and cost is the reason some storage systems still use SLC devices. The durability of SLC devices makes them ideal for write-intensive transaction processing workloads. However, the new classes of applications in machine learning, big data analytics and streaming media involve an increasing number of workloads that predominantly read data rather than writing it, minimizing the importance of flash durability.
QLC vs. TLC: The trend toward fewer writes per day
According to research firm Forward Insights, fewer than 20% of SSDs sold last year were spec'd at more than one drive write per day (DWPD). By 2023, estimates are that 85% of drives sold will be the low-duration models spec'd at one or less DWPD. A DWPD measures the total amount of data written to a drive in proportion to its total capacity and is used to specify guaranteed drive endurance of five years. A 1 TB drive spec'd at one DWPD can sustain an average of 1 TB of data writes every day for five years.
The trend toward fewer writes per day bolsters the case for QLC drives. They have the shortest endurance of any current flash device because of tight tolerances on the charge levels for each bit state in a memory cell along with tighter spacings and thinner insulating gates in leading-edge flash manufacturing processes.
You must be careful when comparing DWPD specs across device types, because it's a relative measure that's a function of total drive capacity. As Micron Technology points out, a 960 GB TLC SSD rated at 1 DWPD has similar overall endurance to a 1.92 TB QLC SSD rated at 0.5 DWPD for a particular workload. Although the QLC DPWD spec is lower, the total amount of data that can be written per day is the same for the two devices. Thus, for workloads that almost exclusively read data, the QLC device is a better choice because of its increased capacity.
How QLC and TLC complement each other
The QLC market is focused on read-dominant workloads; it's not trying to displace TLC devices, but rather to replace HDDs. This is Micron's justification for continuing to manufacture both types of devices. Micron has made a compelling argument that TLC and QLC are complementary, with QLC filling the gaps between TLC flash and HDD magnetic disks. Indeed, Micron makes the case that because SSDs don't wear when reading data, whereas HDDs do, QLC endurance is superior to HDDs for workloads where the data write pattern includes a high percentage of large sequential transfers.
We can more precisely delineate the workload characteristics best suited to QLC vs. TLC drives by looking closely at the I/O patterns of various applications. According to Micron modeling, QLC drives are best for read-intensive workloads until the read/write mix reaches about 70/30 for small or random data transfers, with the ratio reaching 50-50 for applications with large sequential writes. Conversely, TLC drives are better for write-dominant workloads, except for the minority of heavily transactional applications that might require an SLC drive that can handle 5 or 10 DWPD.
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Fortuitously, many of the enterprise applications experiencing the fastest growth and highest uptake have a preponderance of data reads compared to writes. These include:
- data analytics using data lakes or distributed big data applications like Hadoop;
- AI applications using machine or deep learning;
- NoSQL databases;
- large object stores using Ceph, Gluster, Luster and others; and
- streaming media and content delivery networks.
System design matters, too
The design of QLC devices, which use larger memory blocks than TLC, suits I/O with large-block sequential transfers, but not small random I/O typical of many databases.
One way storage systems can work around this limitation is by coupling an array of QLC SSDs with a non-volatile DIMM (NVDIMM) write buffer that performs write coalescing. Small random writes are buffered until enough are accumulated to fill a data block and then the system sequentially writes them through to a disk volume or file system as a single transfer. The NVDIMM cache buffer can be battery- or capacitor-backed dynamic RAM modules, non-volatile Optane persistent memory modules or even a high-durability SLC or TLC NVMe drive.
Both Microsoft and Western Digital have detailed an approach for using QLC with NVMe-zoned namespaces. Enterprise storage systems using QLC are available, with Pure Storage releasing the FlashArray//C in the fourth quarter of 2019.
With a sizable advantage in capacity and lower cost per bit, expect QLC devices to proliferate in enterprise data centers alongside TLC and SLC devices. Storage systems will intelligently manage workload placement according to the I/O characteristics and requirements of each application.
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