As a storage expert, I want to make sure you understand the ins and outs of QLC flash. While it’s the latest generation of NAND technology, it’s not always the best choice for every workload. Let’s dive into what makes QLC unique and where it fits in your data center.
To make the most of QLC storage, you need to understand its strengths and limitations. Take our quiz to test your knowledge and ensure you’re using QLC drives where they’ll have the greatest positive impact on your data center.
Table of Contents
Question 1
Which statement is true?
A. QLC NAND works well with any application
B. Never mix generations of NAND in your data center
C. As capacity has increased with each new generation of NAND, so has performance
D. The durability of SLC NAND makes it ideal for write-intensive applications
Answer
D. The durability of SLC NAND makes it ideal for write-intensive applications
Explanation
Not all applications work well with QLC storage; write-intensive ones, particularly, can shorten the life of a QLC device. QLC drives and other NAND SSDs are complementary and can work together in a data center, with TLC, MLC and SLC drives targeting workloads that have higher throughput and endurance requirements. Performance has slowed with each new generation of NAND flash; more bits per cell make erase and write operations take longer. SLC NAND, which is more durable than the other NAND types, is particularly useful for write-intensive applications.
Question 2
How many P/E cycles can the average QLC drive handle?
A. 100,000
B. 10,000
C. 3,000
D. 1,000
E. 300
Answer
D. 1,000
Explanation
The more bits per cells in the NAND flash used in a device, the fewer P/E cycles it can support over its lifetime. This dichotomy occurs because each write operation causes wear on a cell. The more bits in the cell, the greater the wear. As a result, QLC NAND, with its higher bits-per-cell count, experiences more and faster wear than the other types of NAND. Estimates on the number of P/E cycles each NAND type can support are as follows:
- SLC — 100,000
- MLC — 10,000
- TLC — 3,000
- QLC — 1,000
- PLC — 10
Question 3
Which is not a potential advantage of QLC NAND compared with other types of NAND?
A. It enables higher storage capacity
B. It costs less per gigabyte
C. It provides better performance
D. It’s particularly good for read-intensive workloads
Answer
C. It provides better performance
Explanation
Storing four bits per cell, QLC NAND enables higher storage capacities and costs less per gigabyte than other types of NAND. However, QLC does suffer from poor write durability compared with other types of NAND and is best used for read-intensive workloads. And as more bits are added to each cell, performance is slowed because it takes more time to delete all data in a cell in order to use it to write new data.
Question 4
Which is not a problem with QLC storage?
A. Over the lifetime of a QLC SSD, cells begin to degrade, making them unreliable
B. Increasing the bit count per cell makes the drive slower
C. QLC cells wear out faster than other types of NAND
D. The added capacity of these drives takes up more physical space
Answer
D. The added capacity of these drives takes up more physical space
Explanation
The first three answers describe real QLC NAND challenges. As previously mentioned, cells degrade and become unreliable over the lifetime of a drive. QLC cells also wear out faster than other generations of NAND. As with all types of NAND, each write cycle causes some wear to the cell, but with more bits packed into each cell, QLC is subject to greater wear. The higher bit count per cell also is a drag on performance. Cell erasure times provides a good example of how more bits per cell affects performance:
- With SLC drives, cell erasure takes 1.5 to 2 milliseconds;
- MLC takes 3 milliseconds;
- TLC cells takes 5 milliseconds; and
- QLC takes more than 6 milliseconds.
However, while QLC drives have more capacity than other types of NAND, they don’t take up more space.
Question 5
QLC NAND is superior to TLC in all situations.
A. True
B. False
Answer
B. False
Explanation
While QLC NAND provides a denser medium, storing four bits per cell compared with TLC’s three bits per cell, that doesn’t make it the better option in every situation. QLC storage devices do enable higher capacities at a lower cost per gigabyte. However, they also pose write durability issues and don’t perform as well as TLC NAND drives.
Question 6
Which is a reason that QLC and TLC NAND are complementary technologies?
A. QLC NAND is more likely to replace HDDs than TLC drives
B. They can be combined into one SSD
C. They both have high endurance ratings
D. QLC drives can tolerate a high number of P/E cycles
E. QLC NAND increased density by 50% over TLC NAND
Answer
A. QLC NAND is more likely to replace HDDs than TLC drives
Explanation
The first statement is true. QLC drives fill the gap between TLC drives and HDDs and make a good HDD replacement for read-intensive applications. The other statements are not true. For now, different generations of NAND — SLC, MLC, TLC and QLC — can’t be combined in one drive. Both QLC and TLC NAND have lower endurance ratings and shorter lifespans compared with MLC and SLC. QLC drives can’t tolerate a high number of P/E cycles. QLC NAND increased density over TLC by 33%.
Question 7
Which of the following is an advantage of QLC storage compared with HDDs?
A. It requires less space
B. It delivers better performance
C. It consumes less energy
D. It’s around the same price point
E. All the above
Answer
E. All the above
Explanation
The higher capacity of QLC NAND drives make them a good HDD replacement. They also provide better IOPS, occupy less space, consume less energy and all at a lower cost per gigabyte than other types of NAND.
Question 8
Which type of workload is well suited for QLC?
A. Streaming media
B. Machine learning
C. Big data analytics
D. Active archive
E. All the above
Answer
E. All the above
Explanation
The best applications for QLC NAND are read-intensive ones where writes make up a minimal portion of the overall workload. Audio and video streaming services qualify. Analytics, AI, machine learning and deep learning do, as well, because data is typically written once and then used to carry out sophisticated analytics or learning and training functions. Archiving is another good QLC workload because it involves minimal rewriting of data.
Question 9
What technique isn’t used to extend the life of QLC drives?
A. Overprovisioning
B. Reformatting
C. Wear leveling
D. Error correction code
E. Trim command
Answer
B. Reformatting
Explanation
Overprovisioning an SSD is one of the main ways to extend the life of a NAND drive by mitigating the negative effects of the P/E cycles. Additional capacity is added to the drive above the capacity made available to the user and is used to improve data management. Wear leveling is another important tool for extending SSD endurance. It distributes write cycles across all blocks in a drive, ensuring no one block gets overused. Error correction code is used to correct bit errors, and trim command makes it possible for SSDs to delete old data and do garbage collection at the page rather than block level. Both error correction code and trim command increase drive endurance. Reformatting an SSD means deleting data and resetting partitions on the drive. The reformatting process won’t extend the life of the drive.
Question 10
What’s the next type of NAND after QLC?
A. Penta-level cell, or PLC
B. Quint-level cell, or QTLC
C. Cinq-level cell, or CLC
D. There isn’t any NAND level planned after QLC
Answer
A. Penta-level cell, or PLC
Explanation
NAND with five bits per cell is called penta-level cell. A handful of vendors are working on developing PLC technology, including Intel, Toshiba and Western Digital. It isn’t expected to show up in products for several years.