I added SSD caching to my NAS and learned why it's not the performance fix everyone claims

Published Sep 15, 2026, 5:00 PM EDT Dhruv Bhutani has been writing about consumer technology since 2008, offering deep insights into the personal technology landscape through features and opinion pieces. He writes for XDA-Developers, where he focuses on topics like productivity, networking, self-hosting, and more. Over the years, his work has also appeared in leading publications such as Android Police, Android Authority, CNET, PCMag, and more. Outside of his professional work, Dhruv is an avid fan of horror media spanning films and literature, enjoys fitness activities, collects vinyl records, and plays the guitar. A few months back before RAM prices hit the roof, I decided to add a nice little perk to my NAS. Specifically, I added in SSD caching to eke out a bit more speed, because I thought my hard drives were being the bottleneck to peak performance. My NAS handles everything from file storage to applications, indexing, backups, and more. Once in a while, I might even dabble in a workload with plenty of reads and writes. An SSD cache should help with all that, right? Well, maybe not quite as much as you might think. But that's not because the SSD cache is useless. In fact, the SSD cache is designed specifically to reduce latency for workloads that involve frequent random access to data blocks. But that's just it. A lot of what you actually use a NAS for all day long does not consist of rapid access to small data blocks, and you don't really see any benefit. Let me explain. SSD caching sounds great in theory My NAS workload is mostly large file transfers Conceptually, SSD caching makes a lot of sense. The idea is that the NAS caches frequently accessed data blocks on much faster SSD storage. This means that instead of making every request wait for the hard disk, the SSD can serve the same data much faster when it's requested again. More specifically, a read-write cache handles quick-access tasks and automatically writes the data back to the HDD volume. In principle, it makes a lot of sense. But you need to see how you're actually using your NAS to see why its efficacy varies dramatically based on how you actually use your machine. Turns out, a very significant portion of my workload involves moving large files from one volume to another. My media library, for example, is chock full of video files, and when I move files and folders, these are generally large files moved once, or at best a couple of times. These are sequential operations where the NAS is reading or writing large, contiguous chunks of data, rather than jumping across small bits of data. Ironically, most of us omit reading Synology's own recommendations regarding SSD caches. The company's own documentation mentions that the feature is specifically designed for frequently accessed things like databases, virtual machines, web servers, mail servers, and even snapshots handling a large amount of small data. Everyday use cases like transferring large files or streaming video simply don't benefit from SSD caching. Where SSD caching actually helps Databases and virtual machines can benefit Since caching is more or less an automated process, the system decides which files are worth caching to the SSD. When you stream a large video file, your NAS sees no value in caching that file because the probability of you rewatching it again shortly after is extremely low — forget about watching it hundreds of times a day, which is the kind of workload that SSD caching is designed for. But that doesn't mean an SSD cache serves no purpose. My NAS also serves as my home server. These apps running on my NAS have a very different data access pattern compared to large-volume storage. An app might repeatedly read configuration files, databases, thumbnails, metadata, or other similar information. This is the kind of data that benefits from an SSD cache. Databases, for example, can have a tremendous amount of read and write cycles in a short period of time. Having them cached on the SSD goes a long way towards reducing latency and speeding up access. Similarly, virtual machines can have a large number of I/O operations that put stress on hard drives compared to sequential transfers. If that's your use case, an SSD cache might make sense. But that's just not what I use my NAS for. If you're convinced that an SSD cache is for you, it's worth knowing more about read-only and read-write caches. The two differ fundamentally. As the name suggests, a read-only cache can only access a specific data set but not make any changes to it. Your hard drive continues to be the source of truth. A read-write cache, on the other hand, is basically like a scratch drive. When your workload is constantly modifying data blocks, the quick access work happens at the SSD layer, and all those changes and modifications are then fed downstream to the hard disk. Since this is an inherently risky operation, it is recommended to have a redundant SSD configuration before enabling it. In fact, Synology mandates an SSD RAID setup to enable the read-write cache. It's also worth considering that the SSD cache itself consumes some system resources. You should factor in half a MB of system memory for every 1GB of SSD cache. That's a tiny amount in isolation, but caching hundreds of gigabytes of data would obviously add up. All that to say, an SSD cache doesn't immediately benefit most use cases. If you have a workload involving a small hot dataset, you might see a modest bump in file access speeds, but for most normal everyday use cases, there's little to no benefit to be had. SSD caching is not a replacement for faster hard drives If there's one takeaway to be had, it is that an SSD cache isn't a generic all-purpose performance upgrade. Instead, it takes a very targeted approach. There's been a lot of discussion recently around adding an SSD cache as a must-have to improve NAS performance, but the reality is much more nuanced. While gear-maxxers would recommend filling up every bay in the NAS, it is beneficial to first see how you use your own NAS. I'm guilty of the same. While I was under the impression that my NAS use would get a significant speed bump, the reality is that I just don't use my NAS in a way that would benefit. While I do have an SSD cache, and I see some improvements to Home Assistant performance that runs on the same NAS, I wouldn't say that the performance upgrades have been dramatic, and that investment might have been better served on more hard drives. Now that's an upgrade that everyone can benefit from. Synology DS925+ 7/10 CPU AMD Ryzen V1500B Memory 4 GB DDR4 ECC SODIMM Drive Bays 4 x HDD, 2 x NVMe Expansion 1 x USB Type C The Synology DS925 Plus is a compact and efficient 4 bay NAS built for home users and small businesses. It delivers excellent performance, runs DSM 7.2 with a polished and feature rich interface, offers great mobile app support, and works well as a media server. However, it is limited by its strict drive compatibility requirements and forced automatic updates on newer models.