diff --git a/Proxmox/Install/RunEnv.pm b/Proxmox/Install/RunEnv.pm
index d491aac..5e8a6c2 100644
--- a/Proxmox/Install/RunEnv.pm
+++ b/Proxmox/Install/RunEnv.pm
@@ -354,10 +354,13 @@ sub clamp_zfs_arc_max {
 
     return $mib if $mib == 0;
 
-    my $total_mem_mib = get('total_memory');
-    if ($mib > $total_mem_mib) {
-	return $total_mem_mib;
-    } elsif ($mib < $ZFS_ARC_MIN_SIZE_MIB) {
+    # upper limit is total system memory with a GiB headroom for the base system
+    my $total_mem_with_headroom_mib = get('total_memory') - 1024;
+    if ($mib > $total_mem_with_headroom_mib) {
+	$mib = $total_mem_with_headroom_mib; # do not return directly here, to catch < min ARC size
+    }
+    # lower limit is the ARC min size, else ZFS ignores the setting and uses 50% of total memory again
+    if ($mib < $ZFS_ARC_MIN_SIZE_MIB) {
 	return $ZFS_ARC_MIN_SIZE_MIB;
     }
 
diff --git a/test/zfs-arc-max.pl b/test/zfs-arc-max.pl
index c0938f4..401469e 100755
--- a/test/zfs-arc-max.pl
+++ b/test/zfs-arc-max.pl
@@ -61,9 +61,9 @@ my @clamp_tests = (
     # input, total system memory, expected
     [ 0, 4 * 1024, 0 ],
     [ 16, 4 * 1024, 64 ],
-    [ 4 * 1024, 4 * 1024, 4 * 1024 ],
+    [ 4 * 1024, 4 * 1024, 3 * 1024 ],
     [ 4 * 1024, 6 * 1024, 4 * 1024 ],
-    [ 8 * 1024, 4 * 1024, 4 * 1024 ],
+    [ 8 * 1024, 4 * 1024, 3 * 1024 ],
 );
 
 mock_product('pve');