Building a NAS: how many HDDs / SSDs?

I'm building a DIY NAS following pretty much Wolfgang's YouTube video with its efficient build. I've got everything clear except for the selection of disks. I don't have specially high requirements, but the idea is to buy three HDDs (between 12TB ad 16 TB depending on the offers) and configure them in RAIDz1 to have 24TB avalable and 1-drive failure. I'll make backups of some folders on them to an external drive from time to time.

I'm confused about using SSDs. Wolfgang's comments how he uses 1 TB SSD for each HDD, so I'd have to buy three of those to use them as cache, I guess. My question here is: Why three? Isn't just one (SSDs are much faster, so maybe one would suffice) enough? And why 1 TB and not save on this and buy maybe 512 or even 256GB ones if they just are used as a cache??

One of the example configs the video recommends. Why 3 SSDs?

Hope some one can clarify this. I'd like to take advantage of Black Friday offers if I can find some. I'm using https://diskprices.com to monitor prices, but if you have any other good way to find good deals, please let me know. Thank you!

5 comments

Check Out patriot p300 nvme, 128g boot drive for like 10€. I have two of them, one running flawlessly almost 2 years
- Thanks :)
18 TB disks are still the sweet spot price wise it seems. So I would go for that as many as you can afford. The rest depends on what you actually want to do with the NAS. IMHO, you don't need insane amounts of cache, if the NAS shall just serve some media files. The HDDs itself will be plenty of fine. Just one SSD for the system installation should be all you need, IMHO.
Disregard the advice about SSDs. Start with 1 NVMe for boot and only add more if you have a need. Either for running VMs or if additional cache is actually needed.
As it was mentioned, there is no real need for SSD caching unless it is required by your workload. In any case, you should be able to configure caching later if the performance of HDDs wouldn't satisfy your needs.