From ad4d2d41fb692764d67d9de59b417fabe10c2979 Mon Sep 17 00:00:00 2001
From: Simon Gardling <titaniumtown@proton.me>
Date: Fri, 6 Mar 2026 13:44:07 -0500
Subject: [PATCH] zfs: tweak arc settings

---
 modules/zfs.nix     | 18 +++++++++++++-----
 service-configs.nix |  1 +
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/modules/zfs.nix b/modules/zfs.nix
index 29c00c0..0a16ebc 100644
--- a/modules/zfs.nix
+++ b/modules/zfs.nix
@@ -10,10 +10,17 @@
 
   boot.kernelParams =
     let
-      arc_gb = 32;
-      arc_mb = arc_gb * 1000;
-      arc_kb = arc_mb * 1000;
-      arc_b = arc_kb * 1000;
+      # SEE: https://github.com/openzfs/zfs/pull/15437
+      # and https://blog.thalheim.io/2025/10/17/zfs-ate-my-ram-understanding-the-arc-cache/
+      arc_max_gb = (service_configs.gb_ram * 5) / 8;
+      arc_max_mb = arc_max_gb * 1000;
+      arc_max_kb = arc_max_mb * 1000;
+      arc_max_b = arc_max_kb * 1000;
+
+      arc_min_gb = 4;
+      arc_min_mb = arc_min_gb * 1000;
+      arc_min_kb = arc_min_mb * 1000;
+      arc_min_b = arc_min_kb * 1000;
 
       dirty_gb = 8; # Default value is 4GB, helps smooth writes
       dirty_mb = dirty_gb * 1000;
@@ -21,7 +28,8 @@
       dirty_b = dirty_kb * 1000;
     in
     [
-      "zfs.zfs_arc_max=${builtins.toString arc_b}"
+      "zfs.zfs_arc_max=${builtins.toString arc_max_b}"
+      "zfs.zfs_arc_min=${builtins.toString arc_min_b}"
       "zfs.zfs_txg_timeout=120" # longer TXG open time = larger sequential writes
       "zfs.zfs_dirty_data_max=${builtins.toString dirty_b}"
       "zfs.zfs_delay_min_dirty_percent=80" # delay write throttling until 80% dirty (default 60%)
diff --git a/service-configs.nix b/service-configs.nix
index cee64ee..1636b13 100644
--- a/service-configs.nix
+++ b/service-configs.nix
@@ -7,6 +7,7 @@ rec {
   media_group = "media";
 
   cpu_arch = "znver3";
+  gb_ram = 64;
 
   ports = {
     # public