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When you want to pack a NAS device with storage, hard drives are still the best choice for capacity. And if you want to boost performance for multiple users accessing the same network storage simultaneously, without reducing capacity by giving up any drive bays, the latest NAS boxes have an M.2 slot to accommodate an SSD. If you're using Windows Server Storage Spaces Direct, it will automatically set up any NVMe drive as the write cache.
Seagate's new IronWolf 510 is an NVMe SSD specifically designed for this purpose, with dynamic overprovisioning for endurance and data protection, 256-bit AES encryption, a controller with capacitors to give the drive extra time to write data if there's a power loss, and a large DRAM cache -- 2GB on the 1.92TB model we tested. With one full drive write per day and a five-year warranty, the IronWolf 510 should have about three times the endurance of a typical desktop NVMe drive.
Testing with CrystalDiskMark using 1GiB files came close to the promised 3GB/s read and 1GB/s write speeds on long sequential operations (as you'd expect with the size of the DRAM cache), which is more than enough performance to cache a NAS full of hard drives: in fact, it could keep up with an all-flash NAS using SSDs as those become more affordable, and the excellent endurance means the IronWolf drive should be something you can transfer to a new system that you invest in.
But real-world performance will likely be different, especially for larger files or those not in the DRAM cache. When we tested with an 8GiB file, sequential writes remained around 1GB/s, sustaining performance better than the Intel Optane SSD (tested as an NVMe drive) that we used for comparison, but the read speed dropped considerably for sequential data. With many small random reads or writes, storage performance is always much less efficient: the IronWolf 510's write performance was a match for Optane, but read performance was just over half that of Optane.
Because it's designed for caching, the IronWolf won't be as useful in other scenarios. Testing it as a boot drive in an Intel NUC file server accessed by multiple users and devices on the network didn't work well, for example. However, that was due to a firmware bug in the NUC motherboard that caused it to refuse to boot with an NVMe drive it measured as slow, rather than any issues with the drive itself.
At £459/$519.99, the 1.92TB model isn't cheap although (there are more affordable 240GB, 480GB and 960GB options and it's certainly cheaper than the M.2 SSDs Synology offers for caching). If you compare it to a consumer SSD you might think the write speed is on the low side, but that's not what it's intended for.
This is very specifically a way to get better performance out of a NAS that you've got maxed out on capacity to serve multiple users who are complaining that they want faster access, especially to a lot of files stored in the same folder. Profile your network storage first: it's not likely to make any difference on other NAS tasks like rebuilding the array, but if this kind of cache will improve productivity, the high price tag will be worth it.