The truth about queue depth and why your SSD is already fast enough
Queue depth is one of those things related to SSDs that many people don’t understand or care about, but ignoring it isn’t a good idea. This is why a drive can have serious bragging rights in benchmark tests, while remaining identical to an older generation in everyday use.
Once you know how queue depth works and determine when your SSD actually hits deep queues, you’ll have an easier time choosing an SSD that’s right for your needs…and you may avoid the fastest SSDs.
Queue depth is the hidden trick up your SSD’s sleeve
For better or for worse
Let’s start by finding out how deep the queue is. Queue depth is simply the number of storage requests your system has “in flight” at the same time. If the queue depth is 1, the reader only processes one request at a time. But if it’s at, say, 32, the reader is backlogged and may be processing multiple requests at the same time.
This is important because modern NVMe SSDs are designed for parallelism. In fact, that’s where they thrive. NVMe can accept a lot running commands and keep the controller and NAND busy without slowing down. By the way, that’s what lets you get those huge benchmark numbers on sequential read speeds, but it’s not what entirely determines the day-to-day performance of your SSD.
With a low queue depth, the SSD doesn’t really have a chance to stretch its legs and the benchmark numbers are much less impressive. But it is not only the criteria that make it possible to see this; This also shows up in everyday work and makes the difference between a fast PCIe Gen 5 SSD and a Gen 3 drive much less noticeable than you might think.
I’m mainly talking about NVMe SSDs here, because their parallelism skills are unparalleled (I’m sorry). But hard drives also have queue depth. On SATA hard drives, it appears through the native command queue, where the drive can rearrange requests to reduce head movement. This helps, but spinning rust is still limited by mechanical latency, so deeper queues don’t turn an HDD into an SSD.
Why the crazy speeds of SSDs don’t even matter
This is why you don’t need to buy PCIe Gen 5 drives
Queue depth is also why playback speed can be a misleading specification. Sequential read speed is usually measured as a long, tidy stream, which is ideal for copying a large file, but it does not describe a large number of small, scattered reads occurring at the same time, and this is what happens most often during regular use. Queue depth is about concurrency, and higher queue depth tends to increase IOPS and throughput on mixed or random workloads, while QD1 is where latency and responsiveness are the whole story.
Most everyday computing tasks don’t generate long queues. Opening apps, starting the operating system, using the browser, and pulling in lots of game assets at once are more on the side of burstiness and latency, with the system waiting for one request to complete before throwing a dozen more at the player. This means that a drive that is technically twice as fast at QD32 can have the same feel when you are at a shallow queue depth.
Even if you’re moving loads and loads of data, your SSD is rarely the only factor and bottleneck in how quickly this can happen.
Gaming is a good example of this. Your SSD is constantly working in the background, but it’s not alone; a lot of the workload involves communicating with the CPU and GPU, decompressing, compiling shaders, etc., but the reality is that a lot of the load time is actually just due to your system preparing assets that have already been played. And on Windows you have plenty of caching, so repeat reads can come from RAM instead of the SSD. This is exactly why unstable RAM can also corrupt data.
This is why PCIe Gen 5 drives are so easy to sell. They look amazing in tests that keep them constantly powered and queued, but most people don’t spend their day doing sustained, deep-queued I/O.
Workloads where deep queues and high-end SSDs really pay off
Do you really need a good SSD?
Long queues appear when you are doing work that creates a huge pile of concurrent I/O. Multitasking is really central, as well as many small files or applications that run multiple threads. In these situations, a faster controller, better NAND, and higher QD-IOPS can really matter. Long story short, all that fun stuff that costs more (and it’s not like SSDs are cheap right now to begin with).
Content creation is probably one of the most obvious use cases for these expensive SSDs with deep QDs. If you clean up high-speed footage or work with high-resolution photo catalogs, this difference may stand out. However, most users barely notice it.
The other big “worth it” is workstation-style workloads that behave more like a server. Virtual machines, Docker containers, local databases, large code compilations, and heavy data processing… all of these can really add up on those storage demands.
What specs really matter?
If you’re actually buying an SSD in this economy, make it count
Honestly, I know a lot of people are choosing not to buy SSDs right now, and that’s totally fair. But if you’re interested in buying one, or if you’re reading this in the future when SSD prices have normalized (fingers crossed), here are the specs you should actually be interested in.
If you just want an SSD that feels good every day, don’t obsess over maximum sequential read speed. Read reviews to learn about the performance and consistency of Reduced Queue. Assessments that include random performance at QD1, or other low QD tests, are worth their weight in gold. Beyond that, look at sustained write behavior to learn more about how fast an SSD will be in everyday use, as opposed to exceeding a vendor-specific benchmark.
Then it’s about what you’d expect: capacity, warranty and endurance. In reality, most users consider capacity first and everything else second, and that’s how it should be.
You don’t need a better SSD
If your SSD is working fine, you probably don’t need a faster drive. If your workloads require high QDs and blazing speeds, that’s great. But most of us don’t need to overinvest, which is good news because buying an SSD is currently a pain.
