diff --git a/tests/qemu-iotests/179 b/tests/qemu-iotests/179
new file mode 100755
index 0000000..7bc8db8
--- /dev/null
+++ b/tests/qemu-iotests/179
@@ -0,0 +1,130 @@
+#!/bin/bash
+#
+# Test case for write zeroes with unmap
+#
+# Copyright (C) 2017 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# creator
+owner=eblake@redhat.com
+
+seq="$(basename $0)"
+echo "QA output created by $seq"
+
+here="$PWD"
+status=1	# failure is the default!
+
+_cleanup()
+{
+	_cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+_supported_fmt qcow2
+_supported_proto file
+_supported_os Linux
+
+# v2 images can't mark clusters as zero
+_unsupported_imgopts compat=0.10
+
+echo
+echo '=== Testing write zeroes with unmap ==='
+echo
+
+TEST_IMG="$TEST_IMG.base" _make_test_img 64M
+_make_test_img -b "$TEST_IMG.base"
+
+# Offsets chosen at or near 2M boundaries so test works at all cluster sizes
+# 8k and larger (smaller clusters fail due to non-contiguous allocations)
+
+# Aligned writes to unallocated cluster should not allocate mapping, but must
+# mark cluster as zero, whether or not unmap was requested
+$QEMU_IO -c "write -z -u 2M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z 6M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG.base" | _filter_qemu_img_map
+
+# Unaligned writes need not allocate mapping if the cluster already reads
+# as zero, but must mark cluster as zero, whether or not unmap was requested
+$QEMU_IO -c "write -z -u 10485761 2097150" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z 14680065 2097150" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG.base" | _filter_qemu_img_map
+
+# Requesting unmap of normal data must deallocate; omitting unmap should
+# preserve the mapping
+$QEMU_IO -c "write 18M 14M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z -u 20M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z 24M 6M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG.base" | _filter_qemu_img_map
+
+# Likewise when writing on already-mapped zero data
+$QEMU_IO -c "write -z -u 26M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z 28M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG.base" | _filter_qemu_img_map
+
+# Writing on unmapped zeroes does not allocate
+$QEMU_IO -c "write -z 32M 8M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z -u 34M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z 36M 2M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG.base" | _filter_qemu_img_map
+
+# Writing zero overrides a backing file, regardless of backing cluster type
+$QEMU_IO -c "write -z 40M 8M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write 48M 8M" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "write -z -u 42M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "write -z 44M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "write -z -u 50M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "write -z 52M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "write -z -u 58M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "write -z 60M 2M" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "map" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
+
+# Final check that mappings are correct and images are still sane
+TEST_IMG="$TEST_IMG.base" _check_test_img
+_check_test_img
+
+echo
+echo '=== Testing cache optimization ==='
+echo
+
+BLKDBG_TEST_IMG="blkdebug:$TEST_DIR/blkdebug.conf:$TEST_IMG.base"
+
+cat > "$TEST_DIR/blkdebug.conf" <<EOF
+[inject-error]
+event = "l2_update"
+errno = "5"
+immediately = "on"
+once = "off"
+EOF
+
+# None of the following writes should trigger an L2 update, because the
+# cluster already reads as zero, and we don't have to change allocation
+$QEMU_IO -c "w -z -u 20M 2M" "$BLKDBG_TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "w -z 20M 2M" "$BLKDBG_TEST_IMG" | _filter_qemu_io
+$QEMU_IO -c "w -z 28M 2M" "$BLKDBG_TEST_IMG" | _filter_qemu_io
+
+# success, all done
+echo '*** done'
+status=0
diff --git a/tests/qemu-iotests/179.out b/tests/qemu-iotests/179.out
new file mode 100644
index 0000000..80722b2
--- /dev/null
+++ b/tests/qemu-iotests/179.out
@@ -0,0 +1,156 @@
+QA output created by 179
+
+=== Testing write zeroes with unmap ===
+
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 2097152/2097152 bytes at offset 2097152
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 6291456
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2 MiB (0x200000) bytes not allocated at offset 0 bytes (0x0)
+2 MiB (0x200000) bytes     allocated at offset 2 MiB (0x200000)
+2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
+2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
+56 MiB (0x3800000) bytes not allocated at offset 8 MiB (0x800000)
+[{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
+wrote 2097150/2097150 bytes at offset 10485761
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097150/2097150 bytes at offset 14680065
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2 MiB (0x200000) bytes not allocated at offset 0 bytes (0x0)
+2 MiB (0x200000) bytes     allocated at offset 2 MiB (0x200000)
+2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
+2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
+2 MiB (0x200000) bytes not allocated at offset 8 MiB (0x800000)
+2 MiB (0x200000) bytes     allocated at offset 10 MiB (0xa00000)
+2 MiB (0x200000) bytes not allocated at offset 12 MiB (0xc00000)
+2 MiB (0x200000) bytes     allocated at offset 14 MiB (0xe00000)
+48 MiB (0x3000000) bytes not allocated at offset 16 MiB (0x1000000)
+[{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
+wrote 14680064/14680064 bytes at offset 18874368
+14 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 20971520
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 6291456/6291456 bytes at offset 25165824
+6 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2 MiB (0x200000) bytes not allocated at offset 0 bytes (0x0)
+2 MiB (0x200000) bytes     allocated at offset 2 MiB (0x200000)
+2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
+2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
+2 MiB (0x200000) bytes not allocated at offset 8 MiB (0x800000)
+2 MiB (0x200000) bytes     allocated at offset 10 MiB (0xa00000)
+2 MiB (0x200000) bytes not allocated at offset 12 MiB (0xc00000)
+2 MiB (0x200000) bytes     allocated at offset 14 MiB (0xe00000)
+2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
+14 MiB (0xe00000) bytes     allocated at offset 18 MiB (0x1200000)
+32 MiB (0x2000000) bytes not allocated at offset 32 MiB (0x2000000)
+[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+{ "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 25165824, "length": 6291456, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+wrote 2097152/2097152 bytes at offset 27262976
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 29360128
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2 MiB (0x200000) bytes not allocated at offset 0 bytes (0x0)
+2 MiB (0x200000) bytes     allocated at offset 2 MiB (0x200000)
+2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
+2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
+2 MiB (0x200000) bytes not allocated at offset 8 MiB (0x800000)
+2 MiB (0x200000) bytes     allocated at offset 10 MiB (0xa00000)
+2 MiB (0x200000) bytes not allocated at offset 12 MiB (0xc00000)
+2 MiB (0x200000) bytes     allocated at offset 14 MiB (0xe00000)
+2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
+14 MiB (0xe00000) bytes     allocated at offset 18 MiB (0x1200000)
+32 MiB (0x2000000) bytes not allocated at offset 32 MiB (0x2000000)
+[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+{ "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 25165824, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 27262976, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 29360128, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+wrote 8388608/8388608 bytes at offset 33554432
+8 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 35651584
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 37748736
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2 MiB (0x200000) bytes not allocated at offset 0 bytes (0x0)
+2 MiB (0x200000) bytes     allocated at offset 2 MiB (0x200000)
+2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
+2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
+2 MiB (0x200000) bytes not allocated at offset 8 MiB (0x800000)
+2 MiB (0x200000) bytes     allocated at offset 10 MiB (0xa00000)
+2 MiB (0x200000) bytes not allocated at offset 12 MiB (0xc00000)
+2 MiB (0x200000) bytes     allocated at offset 14 MiB (0xe00000)
+2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
+22 MiB (0x1600000) bytes     allocated at offset 18 MiB (0x1200000)
+24 MiB (0x1800000) bytes not allocated at offset 40 MiB (0x2800000)
+[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+{ "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 25165824, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 27262976, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 29360128, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+wrote 8388608/8388608 bytes at offset 41943040
+8 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 8388608/8388608 bytes at offset 50331648
+8 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 44040192
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 46137344
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 52428800
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 54525952
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 60817408
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 62914560
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+42 MiB (0x2a00000) bytes not allocated at offset 0 bytes (0x0)
+4 MiB (0x400000) bytes     allocated at offset 42 MiB (0x2a00000)
+4 MiB (0x400000) bytes not allocated at offset 46 MiB (0x2e00000)
+4 MiB (0x400000) bytes     allocated at offset 50 MiB (0x3200000)
+4 MiB (0x400000) bytes not allocated at offset 54 MiB (0x3600000)
+4 MiB (0x400000) bytes     allocated at offset 58 MiB (0x3a00000)
+2 MiB (0x200000) bytes not allocated at offset 62 MiB (0x3e00000)
+[{ "start": 0, "length": 18874368, "depth": 1, "zero": true, "data": false},
+{ "start": 18874368, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 20971520, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 23068672, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 25165824, "length": 2097152, "depth": 1, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 27262976, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 29360128, "length": 2097152, "depth": 1, "zero": true, "data": false, "offset": OFFSET},
+{ "start": 31457280, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 33554432, "length": 10485760, "depth": 1, "zero": true, "data": false},
+{ "start": 44040192, "length": 4194304, "depth": 0, "zero": true, "data": false},
+{ "start": 48234496, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 50331648, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 52428800, "length": 4194304, "depth": 0, "zero": true, "data": false},
+{ "start": 56623104, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
+{ "start": 58720256, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 60817408, "length": 4194304, "depth": 0, "zero": true, "data": false},
+{ "start": 65011712, "length": 2097152, "depth": 1, "zero": true, "data": false}]
+No errors were found on the image.
+No errors were found on the image.
+
+=== Testing cache optimization ===
+
+wrote 2097152/2097152 bytes at offset 20971520
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 20971520
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 29360128
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index bb6df8f..5c8ea0f 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -171,5 +171,6 @@
 175 auto quick
 176 rw auto backing
 177 rw auto quick
+179 rw auto quick
 181 rw auto migration
 182 rw auto quick
