Swap: Difference between revisions

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This is probably because the writeback device has already been mounted elsewhere (e.g. as swap). To avoid this you need to do as the [[#Disable swap]] section says and make sure your writeback device is not being mounted as swap (this can happen due to <code>systemd-gpt-auto-generator(8)</code>). Do note that zram writeback does ''not'' respect the swap on-disk format and will destroy your existing swap header.

== Zswap swap cache ==

[https://docs.kernel.org/admin-guide/mm/zswap.html Zswap] is a compressed RAM cache for swap pages. It acts as a middle layer between system memory and a traditional disk-based swap device, storing compressed pages in RAM before optionally writing them out to disk-based swap if necessary. It acts as an LRU cache.

Unlike zram, zswap requires a disk-based swap device or file to back it.

Zswap is controlled by kernel parameters and can be enabled in your NixOS configuration by setting appropriate options through <code>boot.kernelParams</code>.

{{file|/etc/nixos/configuration.nix|nix|

boot.kernelParams = [

"zswap.enabled=1" # enables zswap

"zswap.compressor=lz4" # compression algorithm

"zswap.max_pool_percent=20" # maximum percentage of RAM that zswap is allowed to use

"zswap.shrinker_enabled=1" # whether to shrink the pool proactively on high memory pressure

];

</nowiki>

}}

{{note|If you use the <code>lz4</code> algorithm, you will also need to set {{nixos:option|boot.initrd.systemd.enable}} to true}}

You can verify zswap's runtime status via <code>cat /sys/module/zswap/parameters/enabled</code> and inspect usage statistics with <code># grep -r . /sys/kernel/debug/zswap/</code>

== Disable swap ==

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 This is probably because the writeback device has already been mounted elsewhere (e.g. as swap). To avoid this you need to do as the [[#Disable swap]] section says and make sure your writeback device is not being mounted as swap (this can happen due to <code>systemd-gpt-auto-generator(8)</code>). Do note that zram writeback does ''not'' respect the swap on-disk format and will destroy your existing swap header.
+== Zswap swap cache ==
+[https://docs.kernel.org/admin-guide/mm/zswap.html Zswap] is a compressed RAM cache for swap pages. It acts as a middle layer between system memory and a traditional disk-based swap device, storing compressed pages in RAM before optionally writing them out to disk-based swap if necessary. It acts as an LRU cache.
+Unlike zram, zswap requires a disk-based swap device or file to back it.
+Zswap is controlled by kernel parameters and can be enabled in your NixOS configuration by setting appropriate options through <code>boot.kernelParams</code>.
+{{file|/etc/nixos/configuration.nix|nix|
+<nowiki>
+  boot.kernelParams = [
+    "zswap.enabled=1" # enables zswap
+    "zswap.compressor=lz4" # compression algorithm
+    "zswap.max_pool_percent=20" # maximum percentage of RAM that zswap is allowed to use
+    "zswap.shrinker_enabled=1" # whether to shrink the pool proactively on high memory pressure
+  ];
+</nowiki>
+}}
+{{note|If you use the <code>lz4</code> algorithm, you will also need to set {{nixos:option|boot.initrd.systemd.enable}} to true}}
+You can verify zswap's runtime status via <code>cat /sys/module/zswap/parameters/enabled</code> and inspect usage statistics with <code># grep -r . /sys/kernel/debug/zswap/</code>
 == Disable swap ==