fix #4829: install: add new ZFS arc_max setup option

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>

Proxmox/Install.pm

@@ -291,6 +291,19 @@ sub get_zfs_raid_setup {
     return ($devlist, $cmd);
 }
 
+# If the maximum ARC size for ZFS was explicitly changed by the user, applies
+# it to the new system by setting the `zfs_arc_max` module parameter in /etc/modprobe.d/zfs.conf
+my sub zfs_setup_module_conf {
+    my ($targetdir) = @_;
+
+    my $arc_max = Proxmox::Install::Config::get_zfs_opt('arc_max');
+    my $arc_max_mib = Proxmox::Install::RunEnv::clamp_zfs_arc_max($arc_max) * 1024 * 1024;
+
+    if ($arc_max > 0) {
+	file_write_all("$targetdir/etc/modprobe.d/zfs.conf", "options zfs zfs_arc_max=$arc_max\n")
+    }
+}
+
 sub get_btrfs_raid_setup {
     my $filesys = Proxmox::Install::Config::get_filesys();
 
@@ -1141,6 +1154,7 @@ _EOD
 
 	    file_write_all("$targetdir/etc/kernel/cmdline", "root=ZFS=$zfs_pool_name/ROOT/$zfs_root_volume_name boot=zfs\n");
 
+	    zfs_setup_module_conf($targetdir);
 	}
 
 	diversion_remove($targetdir, "/usr/sbin/update-grub");

Proxmox/Install/Config.pm

@@ -72,6 +72,7 @@ my sub init_cfg {
 	    compress => 'on',
 	    checksum => 'on',
 	    copies => 1,
+	    arc_max => Proxmox::Install::RunEnv::default_zfs_arc_max(), # in MiB
 	},
 	# TODO: single disk selection config
 	target_hd => undef,

Proxmox/Install/RunEnv.pm

@@ -303,6 +303,51 @@ sub query_installation_environment : prototype() {
     return $output;
 }
 
+# OpenZFS specifies 64 MiB as the absolute minimum:
+# https://openzfs.github.io/openzfs-docs/Performance%20and%20Tuning/Module%20Parameters.html#zfs-arc-max
+our $ZFS_ARC_MIN_SIZE_MIB = 64; # MiB
+
+# See https://bugzilla.proxmox.com/show_bug.cgi?id=4829
+our $ZFS_ARC_MAX_SIZE_MIB = 16 * 1024; # 16384 MiB = 16 GiB
+our $ZFS_ARC_SYSMEM_PERCENTAGE = 0.1; # use 10% of available system memory by default
+
+# Calculates the default upper limit for the ZFS ARC size.
+# Returns the default ZFS maximum ARC size in MiB.
+sub default_zfs_arc_max {
+    # Use ZFS default on non-PVE
+    return 0 if Proxmox::Install::ISOEnv::get('product') ne 'pve';
+
+    my $default_mib = get('total_memory') * $ZFS_ARC_SYSMEM_PERCENTAGE;
+    my $rounded_mib = int(sprintf('%.0f', $default_mib));
+    print "total_memory:" . get('total_memory') . " mib_rounded:$rounded_mib\n";
+
+    if ($rounded_mib > $ZFS_ARC_MAX_SIZE_MIB) {
+	return $ZFS_ARC_MAX_SIZE_MIB;
+    } elsif ($rounded_mib < $ZFS_ARC_MIN_SIZE_MIB) {
+	return $ZFS_ARC_MIN_SIZE_MIB;
+    }
+
+    return $rounded_mib;
+}
+
+# Clamps the provided ZFS arc_max value (in MiB) to the accepted bounds. The
+# lower is specified by `$ZFS_ARC_MIN_SIZE_MIB`, the upper by the available system memory.
+# Returns the clamped value in MiB.
+sub clamp_zfs_arc_max {
+    my ($mib) = @_;
+
+    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) {
+	return $ZFS_ARC_MIN_SIZE_MIB;
+    }
+
+    return $mib;
+}
+
 my $_env = undef;
 sub get {
     my ($k) = @_;