diff --git a/xCAT-server/lib/xcat/plugins/mknb.pm b/xCAT-server/lib/xcat/plugins/mknb.pm
index 21253dd49..b385f0698 100644
--- a/xCAT-server/lib/xcat/plugins/mknb.pm
+++ b/xCAT-server/lib/xcat/plugins/mknb.pm
@@ -246,6 +246,13 @@ sub _install_prebuilt_genesis {
     return ("$destination_dir/genesis.fs.$arch.gz", undef);
 }
 
+sub genesis_lzma_command {
+    my ($have_lzma, $have_xz) = @_;
+    return 'lzma -C crc32 -9'             if $have_lzma;
+    return 'xz --format=lzma -C crc32 -9' if $have_xz;
+    return;
+}
+
 sub process_request {
     my $request  = shift;
     my $callback = shift;
@@ -484,9 +491,10 @@ sub process_request {
         # place, so concurrent mknb runs sharing $tftpdir cannot read or clobber
         # a half-written genesis.fs.
         my $suffix = xCAT::Utils::genpassword(24);
-        if (-x "/usr/bin/lzma") {    #let's reclaim some of that size...
+        my $lzma_command = genesis_lzma_command(-x "/usr/bin/lzma", -x "/usr/bin/xz");
+        if ($lzma_command) {    #let's reclaim some of that size...
             $callback->({ data => ["Creating genesis.fs.$arch.lzma in $tftpdir/xcat"] });
-            system("cd $tempdir; find . | cpio -o -H newc | lzma -C crc32 -9 > $tftpdir/xcat/genesis.fs.$arch.lzma.$suffix");
+            system("cd $tempdir; find . | cpio -o -H newc | $lzma_command > $tftpdir/xcat/genesis.fs.$arch.lzma.$suffix");
             $lzma_exit_value = $? >> 8;
             if ($lzma_exit_value) {
                 $callback->({ data => ["Creating genesis.fs.$arch.lzma in $tftpdir/xcat failed, falling back to gzip"] });
diff --git a/xCAT-test/unit/mknb_genesis_compressor.t b/xCAT-test/unit/mknb_genesis_compressor.t
new file mode 100644
index 000000000..529f2b74a
--- /dev/null
+++ b/xCAT-test/unit/mknb_genesis_compressor.t
@@ -0,0 +1,54 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use FindBin;
+use File::Spec;
+use Test::More;
+
+my $plugin = File::Spec->catfile( $FindBin::Bin, '..', '..',
+    'xCAT-server', 'lib', 'xcat', 'plugins', 'mknb.pm' );
+plan skip_all => 'mknb.pm not found' unless -r $plugin;
+
+open( my $fh, '<', $plugin ) or die "Unable to read $plugin: $!";
+my $source = do { local $/; <$fh> };
+close($fh);
+
+# mknb.pm needs a management node to load, so lift the routine out and drive
+# the real code on its own.
+my ($routine) = $source =~ /(sub genesis_lzma_command \{.*?\n\}\n)/s;
+BAIL_OUT('could not extract genesis_lzma_command from mknb.pm') unless $routine;
+eval "package MknbCompressor; $routine 1;" or BAIL_OUT("could not evaluate: $@");
+
+sub command { return MknbCompressor::genesis_lzma_command(@_); }
+
+# Debian and Ubuntu ship both names, and lzma is the one the plugin has always
+# used, so nothing changes on those systems.
+is( command( 1, 1 ), 'lzma -C crc32 -9', 'lzma is used when it is there' );
+
+# Red Hat ships xz alone.
+is( command( 0, 1 ), 'xz --format=lzma -C crc32 -9',
+    'xz stands in for lzma when only xz is there' );
+
+# The gzip path below the caller handles a system with neither.
+is( command( 0, 0 ), undef, 'nothing is returned when neither program is there' );
+
+# The container has to stay the one the file name promises. "xz" on its own
+# writes the xz container, which the file name does not describe and which a
+# reader of a .lzma file cannot open.
+my ($xz_form) = command( 0, 1 ) =~ /--format=(\S+)/;
+is( $xz_form, 'lzma', 'xz is asked for the lzma container, not its own' );
+
+# The caller has to take the command from the routine, and the name of the
+# written file must not change with it.
+like( $source, qr/genesis_lzma_command\(-x "\/usr\/bin\/lzma", -x "\/usr\/bin\/xz"\)/,
+    'the caller asks the routine which program to run' );
+like( $source, qr/cpio -o -H newc \| \$lzma_command > \$tftpdir\/xcat\/genesis\.fs\.\$arch\.lzma\.\$suffix/,
+    'the chosen command writes the same file under the same suffix' );
+
+# The fallback to gzip and the atomic rename both have to survive.
+like( $source, qr/falling back to gzip/, 'the gzip fallback is still reported' );
+like( $source, qr/move\("\$tftpdir\/xcat\/genesis\.fs\.\$arch\.lzma\.\$suffix"/,
+    'the finished image is still renamed into place' );
+
+done_testing();