make NixOS more SSD-friendly

[KiaraGrouwstra]

I've installed NixOS on my 128 GB SSD, but every so often find myself running out of space, despite the occasional maintenance.

NixOS solves reproducability at the cost of storing multiple OS revisions.
SSDs give increased performance, but are costlier, meaning they'll generally easily get full.

It'd be great to get the best of both here. I think this would look like having most of /nix/store installed on an HDD, while allowing anything part of the 'active' revision to get copied to an SSD for the performance boost.

I don't know if anyone has considered this so far, or if this should go in Nix instead, but I wanted to just put this out there to gather feedback / thoughts.

[DIzFer]

AFAIK, solving this is as simple as configuring the mountpoint for /nix/store, via fileSystems."/nix/store".*. By its very nature, NixOS doesn't have this kind of defaults, since it's a particular configuration step that depends on user preference and hardware layout. Therefore, there's nothing really to do on the coding side.

However, I think that would be missing the point of having an SSD, since then all your software would be on a slow device. I'd suggest looking into bcache{,fs}, so that it's your kernel the one to decide where to put each bit.

[vcunat]

Yes, but for SSDs it seems best to use nix.autoOptimiseStore = true; – that can be expensive on rotating HDDs, which is possibly why it isn't the default.

[vcunat]

BTW, for combining SSD and a rotating HDD you may want to use general solutions such as bcache.

[KiaraGrouwstra]

That sounds great, I'll try -- thank you!