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QLC edges into mainstream, but what are its advantages?

QLC flash offers high density but has lifecycle limitations. But what does it really cost compared with TLC and MLC, and how are suppliers implementing it and getting around durability issues?

With the recent launches and enhancements to product lines from NetApp and Pure Storage, QLC flash storage has pushed itself further into the spotlight.

QLC is quad-level cell flash storage, and allows for much higher capacity and density in solid-state drives (SSDs).

In this article, we’ll define QLC flash, look at what what’s driving its adoption, and examine the use cases it is good for.

What is QLC flash?

Flash technology has evolved from single-level cell (SLC) and multi-level cell (MLC) flash via triple-level cell (TLC) – all of which indicate the number of charges in a flash cell – to quad-level cell (QLC) flash storage.

QLC means it stores four bits per cell and provides 16 possible binary states, which is how it boosts capacity over previous generations.

But with each new generation of storage, there is an element of “how long can you hang onto the rising balloon?”.

In other words, with all those voltage levels packed into increasingly smaller volumes of silicon, there is scope for more wear and more things to go wrong, which can lead to data corruption, for example.

Storage array makers handle this by building efficiencies into software to even out the use of cells – for example, wear levelling. Also, QLC flash is aimed at use cases that are not likely to be subject to massive volumes of inpute/output (I/O) cycles.

QLC flash use cases

The top-line takeaway about QLC is that it is not well-suited to high I/O situations, so you wouldn’t use it for transactional databases.

It is suited as a storage solution for data that is less frequently accessed, that emphasises reads rather than writes, and with a strong bias towards sequential I/O rather than random.

But being solid state rather than spinning disk means QLC provides much more rapid access than hard disk drives (HDDs) can provide.

Bringing that down to more concrete workload terms, that means secondary storage use cases, such as:

  • Storage for analytics workloads where bulk datasets require quick access.
  • Storage of backups, where the bulk of data will unlikely be accessed but when it is needed it is needed quickly.
  • In particular, in case of ransomware attack a customer may need to restore a lot of data very quickly.

But note that QLC-equipped array products can differ in terms of their I/O, with drives that contain more or less cache to provide products that offer drives that vary in terms of the access times they can deliver.

QLC flash cost

A big attraction of QLC flash is that it allows for higher-density and lower-cost solid-state storage. Some suppliers and analysts have claimed the price per gigabyte (GB) of QLC is on the way to parity with hard disk drives.

It’s difficult to find comprehensive publicly available drive pricing data, but tracks all those that Amazon sells, so it’s possible to crunch that data for SSDs of different types.

At the time of writing, listed 216 new internal SSDs for sale on in capacities up to 8TB. Of these, nine were QLC, 30 were TLC and 159 were not identified, but we can assume most of those are MLC because it is the most prevalent form of flash type.

And in fact, price per gigabyte from this source does accord with the density of storage offered by the NAND types offered:

  • MLC 10 cents per gigabyte.
  • TLC 9.6 cents per gigabyte.
  • QLC 9.2 cents per gigabyte.

By way of comparison with HDDs, 268 new internal 2.5in and 3.5in drives up to 8TB were listed by and these came out at:

  • SAS HDD 5 cents per gigabyte.
  • SATA HDD 3.1 cents per gigabyte.

The latter are listed as HDD, of which most are SATA-connected if you dig into the description.  

That’s a rough guide to raw cost per gigabyte, but doesn’t include total cost of ownership in storage arrays, which may bring down spend further over its lifetime. It also doesn’t account for some of the larger drives – commodity and proprietary – which can bring the price per gigabyte down also.

QLC flash in storage products

Four of the big six storage array makers offer QLC options in products aimed at analytics and secondary use cases. HPE and Hitachi appear to be in wait-and-see mode.

Dell EMC rebranded its Isilon scale-out NAS as PowerScale in 2020 and offers it with QLC drives of up to 30TB in the F900 and F600 all-flash models. It targets those QLC-equipped products at analytics, media, artificial intelligence and machine learning, and deep learning workloads.

IBM can equip its FlashSystem family with QLC drives. It first told the world how it could make QLC behave like TLC back in 2020 by using its FlashCore Modules – which are proprietary QLC drives – to lengthen drive life by monitoring and optimising flash cell health over time.

NetApp recently launched a new QLC flash storage array family – the C-series – aimed at higher-capacity use cases that also need the speed of SSD. NetApp’s C-series starts with three options – the C250, C400 and C800 – which scale to 35PB (petabytes), 71PB and 106PB respectively.

Pure Storage has built its FlashBlade family around QLC flash drives, and these fall into two series, the //S and the //E, which are differentiated from each other by performance. FlashBlade//S packs more cache into the (proprietary) DirectFlash Modules for better I/O, while the //E variant veers more towards capacity than performance.

Outside the big six, there are some suppliers with QLC-based products.

Vast Data only offers arrays based NVMe-connected QLC. To mitigate QLC’s limitations, Vast does its best to sequentialise traffic with a layer of storage-class memory that shapes I/O into fewer, less randomised patterns.

Meanwhile, NexSan offers QLC drive support in its E-series F platforms, up to 15.36TB (terabytes), and Beast Elite F, up to 7.68TB. The E-Series can be used as expansion for NexSan’s Unity unified SAN/NAS array.

Read more about QLC flash storage

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