This allows capping the total amount of memory that will be used for
zram-swap, in addition to the percentage-based calculation, which is
useful when blanket-applying a configuration to many machines.
This is based off the strategy used by Fedora for their rollout of
zram-swap-by-default in Fedora 33
(https://fedoraproject.org/wiki/Changes/SwapOnZRAM), which caps the
maximum amount of memory used for zram at 4GiB.
In future it might be good to port this to the systemd zram-generator,
instead of using this separate infrastructure.
This creates a dependency cycle when used with boot.tmpOnTmpfs:
basic.target <- tmp.mount <- swap.target <- zram-init-dev0 <- basic.target
This same fix is done already for tmp.mount
Fixes https://github.com/NixOS/nixpkgs/issues/47474
- add `zramSwap.algorithm` option, which allows to change compressor
declaratively. zstd as default
- add `zramSwap.swapDevices` option, which allows to define how many zram
devices will be used as swap. Rest devices can be managed freely
- simpler floating calculations
- fix udev race condition
- some documentation changes
- replaced `/sys/block/zram*` handling with `zramctl`, because I had occasional
"Device is busy" error (looks like zram has to be configured in predefined order)
- added `memoryPercent` and `algorithm` as restart triggers. I think, it was
a bug that changing `memoryPercent` in configuration wasn't applied immediately.
- removed a bind to .swap device. While it looks natural (when swap device goes
off, so should zram device), it wasn't implemented properly. This caused problems
with swapon/swapoff:
```
$ cat /proc/swaps
Filename Type Size Used Priority
/dev/zram0 partition 8166024 0 -2
/var/swapfile file 5119996 5120 1
$ sudo swapoff -a
$ sudo swapon -a
swapon: /dev/zram0: read swap header failed
$ cat /proc/swaps
Filename Type Size Used Priority
/var/swapfile file 5119996 0 1
```
This allows you to use the Linux kernel's built-in compressed memory as
swap space functionality.
It is recommended to enable only for kernel 3.14 (which is when zram came out of
the staging drivers area) or higher.
