Disable blkif indirect segment I/Os in EC2 by default due to performance

issues on some EC2 instance types.  Users may want to experiment with
removing this from loader.conf and measuring the performance impact on
the EC2 instances they are using.

release/tools/ec2.conf

@@ -70,6 +70,11 @@ vm_extra_pre_umount() {
 	# nodes, but apply the workaround just in case.
 	echo 'hw.broken_txfifo="1"' >> ${DESTDIR}/boot/loader.conf
 
+	# Some EC2 instances suffer a significant (~40%) reduction in
+	# throughput when using blkif indirect segment I/Os.  Disable this
+	# by default for now.
+	echo 'hw.xbd.xbd_enable_indirect="0"' >> ${DESTDIR}/boot/loader.conf
+
 	# The first time the AMI boots, the installed "first boot" scripts
 	# should be allowed to run:
 	# * ec2_configinit (download and process EC2 user-data)