hydra: postgres tuning

Main thing that seemed to make a difference in pgbench was
full_page_writes=off

modules/config/hydra.nix

@@ -12,13 +12,25 @@
     useSubstitutes = true;
   };
 
+  # ZFS dataset properties:
+  # atime=off
+  # compression=zstd
+  # recordsize=16K
   services.postgresql = {
-    package = pkgs.postgresql_13;
+    package = pkgs.postgresql_17;
     dataDir = "/var/db/postgresql-${config.services.postgresql.package.psqlSchema}";
     settings = {
       max_connections = 250;
       work_mem = "8MB";
-      shared_buffers = "512MB";
+      shared_buffers = "4GB";
+      # ZFS ARC defaults to 50% of RAM
+      effective_cache_size = "64GB";
+      # ZFS writes are atomic, so this is unnecessary
+      # See: https://wiki.postgresql.org/wiki/Full_page_writes
+      full_page_writes = "off";
+      # Useless with CoW
+      wal_init_zero = "off";
+      wal_recycle = "off";
     };
   };
 

modules/tasks/filesystems/zfs.nix

@@ -2,8 +2,9 @@
 { config, lib, pkgs, ... }: with lib; mkIf (config.boot.supportedFilesystems.zfs or false) (mkMerge [
   ({
     boot = {
+      # Use default LTS kernel
       # Lower priority than mkForce to allow devices to use custom kernels
-      kernelPackages = mkOverride 75 config.boot.zfs.package.latestCompatibleLinuxPackages;
+      kernelPackages = mkOverride 75 pkgs.linuxPackages;
       zfs = {
         # Recommended to be disabled to avoid potential corruption
         forceImportRoot = false;