Swap, ZRAM, Zswap and Hibernate on NixOS

This explains how to set up swap, zram, zswap and hibernate on NixOS for desktop and laptop use, with a focus on getting suspend-then-hibernate to work reliably. It emphasizes that hibernation requires disk-backed swap to store the memory image and that swap files versus swap partitions matter because the resume path must be known before filesystems mount; NixOS now understands swap file offsets and can handle UEFI resume metadata. Practical setup examples contrast a simple swap partition on a desktop with an encrypted LUKS + btrfs subvolume swap file on a laptop, and point out that swap files can be made to work but add complexity.

The recommended approach is to use zswap backing a disk-backed swap and to tune the kernel swappiness higher (the author uses 100) rather than using zram (zramSwap), which is inappropriate when you plan to hibernate. A minimal NixOS config enabling zswap and setting swappiness is sufficient. Compression choice matters: kernel defaults to lzo, NixOS defaults to zstd for better ratio, and lz4 is fastest; the tradeoff is speed versus how many pages can stay compressed in RAM (fewer disk writes). Recent NixOS defaults have simplified the needed configuration.