quad-level cell NAND (QLC NAND)

What is quad-level cell NAND (QLC NAND)?

Quad-level cell NAND (QLC NAND) is a form of NAND flash memory that can store up to 4 bits of data per memory cell.

NAND flash memory is a type of non-volatile storage that does not require power to retain data. It can be found in everyday devices -- such as USB flash drives -- and also in enterprise server, storage and networking environments. NAND flash memory typically competes with magnetic storage devices, such as hard disks. To give solid-state drives (SSDs) more storage capacity, storage vendors began developing QLC NAND as a new NAND storage technology.

Normally, the preface used for the different types of NAND flash memory refers to how many bits of data can be stored in a NAND cell; therefore, quad-level cell indicates the storage of 4 bits. Other forms of NAND flash include single-level cell (SLC), multi-level cell (MLC) and triple-level cell (TLC). SLC stores 1 bit per cell, MLC stores 2 bits and TLC stores 3 bits per cell.

Besides offering one more bit of storage per cell than TLC, strengths of QLC NAND include speed and cost. Organizations that should use QLC NAND include companies that work with server-based, read-intensive workloads. Organizations that use older hard disk drives (HDDs) would also benefit significantly from upgrading to QLC NAND. A few ideal use case examples for QLC NAND workloads include streaming from media servers, the use of data analytics tools and the use of machine learning.

Chart of NAND flash forms
NAND flash characteristics differ based on performance, endurance, density, error probability and price.

However, if an organization is doing well with their current NAND options, then an upgrade isn't necessary. Moreover, users who want to upgrade to QLC NAND should keep in mind that it is not fit for write-intensive workloads and is not as durable compared to the other types of storage.

QLC NAND is available from vendors such as Samsung, Micron Technology and Adata Technology.

Features of QLC NAND

Features and characteristics of QLC NAND include the following:

  • The ability to store up to 4 bits of data per cell.
  • Provides a 33% increase in storage capacity per cell compared to TLC.
  • Needs less space.
  • Requires less energy.
  • High density, which can significantly reduce server rack space.
  • Meant for low-intensity workloads -- able to support 100 write cycles.
  • Fast read speeds for read-intensive or performance-based workloads.

Applications of QLC NAND

Nearly any data store that supports read-intensive applications can use QLC. QLC NAND can be used for analytical applications that support artificial intelligence, machine learning, deep learning and other forms of AI. The data in these scenarios is typically written once before carrying out analytics that require fast access to storage with large amounts of data.

Archiving data is another potential application for a QLC workload. This is especially applicable if an organization uses data for AI-based analytics or business intelligence applications that mine data.

Large data centers that support streaming media operations could use QLC to deliver audio or video services faster and more efficiently to their customers.

A NoSQL database full of rich data and metadata could also use QLC NAND flash drives, giving a boost in application performance.

Other potential applications of QLC include the use of real-time analytics and user authentication.

QLC NAND life span

While QLC NAND performs well in terms of speed and storage capacities, it has low writing durability and may be slower compared to other types of NAND flash memory.

Write operations will typically damage NAND cells, and as such, a NAND cell has a set amount of SSD write cycles it can complete before it begins to fail. Therefore, it is not recommended to use QLC NAND in write-intensive workloads.

The number of write operations a NAND cell can take before failing is different for each NAND flash storage type, and therefore, each type will have a different life span. SLC disks support up to around 100,000 write cycles, MLC supports up to about 10,000 write cycles and TLC supports anywhere from 300 to 1,000 write cycles. QLC NAND supports around 100 write cycles.

Making use of 3D NAND -- which stacks cells vertically up to 32 layers -- can increase the number of write cycles. For example, using 3D NAND with MLC increases the number of write cycles from 10,000 to 35,000, and -- when used with TLC -- write cycles increase from around 1,000 to approximately 3,000. Using 3D NAND with QLC NAND can support about 1,000 write cycles, which is better than without 3D NAND, but still falls short when compared to the other options.

However, there are ways for vendors to maneuver around the low number of write operations that QLC NAND can handle. With wear leveling, data can be arranged in such a way that write and erase cycles are distributed equally among memory blocks in a drive. This process extends the life of a QLC NAND drive.

Overprovisioning also extends the life of a QLC NAND drive. With the overprovisioning strategy, extra capacity is added to a drive, but is not made available as storage. The additional capacity enables write and erase cycles to be distributed across a greater amount of NAND flash blocks over time. This process increases the durability of a QLC NAND drive and provides more space to manage data, thus also improving performance.

Pros and cons of QLC NAND

QLC NAND has specific advantages and disadvantages, which admins should consider when deciding whether to implement it in an environment. Pros of QLC NAND include the following:

  • More storage capacity than SLC, MLC and TLC NAND.
  • Fast read speeds.
  • Lower costs per gigabyte.
  • Consumes less power than an HDD.
  • Can improve desktop performance when replacing older HDDs.

Cons of using QLC NAND revolve around its relatively weak endurance. They include the following:

  • The number of write cycles a QLC NAND drive can handle is much less than SLC and MLC drives.
  • QLC NAND drives wear out faster compared to SLC and MLC.
  • There is no life span improvement; it is nearly equal to that of TLC.
  • Capacity increases by less and less with each NAND upgrade.
This was last updated in August 2023

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