diff --git a/block/Makefile.objs b/block/Makefile.objs
index d644bac60a65..899bfb5e2cf0 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -26,7 +26,7 @@ block-obj-y += accounting.o dirty-bitmap.o
 block-obj-y += write-threshold.o
 block-obj-y += backup.o
 block-obj-$(CONFIG_REPLICATION) += replication.o
-block-obj-y += throttle.o
+block-obj-y += throttle.o copy-on-read.o
 
 block-obj-y += crypto.o
 
diff --git a/block/blkdebug.c b/block/blkdebug.c
index 053372c22ea2..526af2a80843 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -398,10 +398,11 @@ static int blkdebug_open(BlockDriverState *bs, QDict *options, int flags,
         goto out;
     }
 
-    bs->supported_write_flags = BDRV_REQ_FUA &
-        bs->file->bs->supported_write_flags;
-    bs->supported_zero_flags = (BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
-        bs->file->bs->supported_zero_flags;
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
+        (BDRV_REQ_FUA & bs->file->bs->supported_write_flags);
+    bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+        ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
+            bs->file->bs->supported_zero_flags);
     ret = -EINVAL;
 
     /* Set alignment overrides */
diff --git a/block/blkreplay.c b/block/blkreplay.c
index fe5a9b4a984d..b016dbeee75a 100755
--- a/block/blkreplay.c
+++ b/block/blkreplay.c
@@ -35,6 +35,9 @@ static int blkreplay_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
+    bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED;
+
     ret = 0;
 fail:
     return ret;
diff --git a/block/blkverify.c b/block/blkverify.c
index 754cc9e85769..da97ee592767 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -141,6 +141,9 @@ static int blkverify_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
+    bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED;
+
     ret = 0;
 fail:
     qemu_opts_del(opts);
diff --git a/block/copy-on-read.c b/block/copy-on-read.c
new file mode 100644
index 000000000000..6a972088889f
--- /dev/null
+++ b/block/copy-on-read.c
@@ -0,0 +1,173 @@
+/*
+ * Copy-on-read filter block driver
+ *
+ * Copyright (c) 2018 Red Hat, Inc.
+ *
+ * Author:
+ *   Max Reitz <mreitz@redhat.com>
+ *
+ * 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 or
+ * (at your option) version 3 of the License.
+ *
+ * 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/>.
+ */
+
+#include "qemu/osdep.h"
+#include "block/block_int.h"
+
+
+static int cor_open(BlockDriverState *bs, QDict *options, int flags,
+                    Error **errp)
+{
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file, false,
+                               errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
+                                (BDRV_REQ_FUA &
+                                    bs->file->bs->supported_write_flags);
+
+    bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+                               ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
+                                    bs->file->bs->supported_zero_flags);
+
+    return 0;
+}
+
+
+static void cor_close(BlockDriverState *bs)
+{
+}
+
+
+#define PERM_PASSTHROUGH (BLK_PERM_CONSISTENT_READ \
+                          | BLK_PERM_WRITE \
+                          | BLK_PERM_RESIZE)
+#define PERM_UNCHANGED (BLK_PERM_ALL & ~PERM_PASSTHROUGH)
+
+static void cor_child_perm(BlockDriverState *bs, BdrvChild *c,
+                           const BdrvChildRole *role,
+                           BlockReopenQueue *reopen_queue,
+                           uint64_t perm, uint64_t shared,
+                           uint64_t *nperm, uint64_t *nshared)
+{
+    if (c == NULL) {
+        *nperm = (perm & PERM_PASSTHROUGH) | BLK_PERM_WRITE_UNCHANGED;
+        *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
+        return;
+    }
+
+    *nperm = (perm & PERM_PASSTHROUGH) |
+             (c->perm & PERM_UNCHANGED);
+    *nshared = (shared & PERM_PASSTHROUGH) |
+               (c->shared_perm & PERM_UNCHANGED);
+}
+
+
+static int64_t cor_getlength(BlockDriverState *bs)
+{
+    return bdrv_getlength(bs->file->bs);
+}
+
+
+static int cor_truncate(BlockDriverState *bs, int64_t offset,
+                        PreallocMode prealloc, Error **errp)
+{
+    return bdrv_truncate(bs->file, offset, prealloc, errp);
+}
+
+
+static int coroutine_fn cor_co_preadv(BlockDriverState *bs,
+                                      uint64_t offset, uint64_t bytes,
+                                      QEMUIOVector *qiov, int flags)
+{
+    return bdrv_co_preadv(bs->file, offset, bytes, qiov,
+                          flags | BDRV_REQ_COPY_ON_READ);
+}
+
+
+static int coroutine_fn cor_co_pwritev(BlockDriverState *bs,
+                                       uint64_t offset, uint64_t bytes,
+                                       QEMUIOVector *qiov, int flags)
+{
+
+    return bdrv_co_pwritev(bs->file, offset, bytes, qiov, flags);
+}
+
+
+static int coroutine_fn cor_co_pwrite_zeroes(BlockDriverState *bs,
+                                             int64_t offset, int bytes,
+                                             BdrvRequestFlags flags)
+{
+    return bdrv_co_pwrite_zeroes(bs->file, offset, bytes, flags);
+}
+
+
+static int coroutine_fn cor_co_pdiscard(BlockDriverState *bs,
+                                        int64_t offset, int bytes)
+{
+    return bdrv_co_pdiscard(bs->file->bs, offset, bytes);
+}
+
+
+static void cor_eject(BlockDriverState *bs, bool eject_flag)
+{
+    bdrv_eject(bs->file->bs, eject_flag);
+}
+
+
+static void cor_lock_medium(BlockDriverState *bs, bool locked)
+{
+    bdrv_lock_medium(bs->file->bs, locked);
+}
+
+
+static bool cor_recurse_is_first_non_filter(BlockDriverState *bs,
+                                            BlockDriverState *candidate)
+{
+    return bdrv_recurse_is_first_non_filter(bs->file->bs, candidate);
+}
+
+
+BlockDriver bdrv_copy_on_read = {
+    .format_name                        = "copy-on-read",
+
+    .bdrv_open                          = cor_open,
+    .bdrv_close                         = cor_close,
+    .bdrv_child_perm                    = cor_child_perm,
+
+    .bdrv_getlength                     = cor_getlength,
+    .bdrv_truncate                      = cor_truncate,
+
+    .bdrv_co_preadv                     = cor_co_preadv,
+    .bdrv_co_pwritev                    = cor_co_pwritev,
+    .bdrv_co_pwrite_zeroes              = cor_co_pwrite_zeroes,
+    .bdrv_co_pdiscard                   = cor_co_pdiscard,
+
+    .bdrv_eject                         = cor_eject,
+    .bdrv_lock_medium                   = cor_lock_medium,
+
+    .bdrv_co_block_status               = bdrv_co_block_status_from_file,
+
+    .bdrv_recurse_is_first_non_filter   = cor_recurse_is_first_non_filter,
+
+    .has_variable_length                = true,
+    .is_filter                          = true,
+};
+
+static void bdrv_copy_on_read_init(void)
+{
+    bdrv_register(&bdrv_copy_on_read);
+}
+
+block_init(bdrv_copy_on_read_init);
diff --git a/block/io.c b/block/io.c
index 4fad5ac2fef6..ca96b487eb84 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1118,13 +1118,15 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
                 /* FIXME: Should we (perhaps conditionally) be setting
                  * BDRV_REQ_MAY_UNMAP, if it will allow for a sparser copy
                  * that still correctly reads as zero? */
-                ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, pnum, 0);
+                ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, pnum,
+                                               BDRV_REQ_WRITE_UNCHANGED);
             } else {
                 /* This does not change the data on the disk, it is not
                  * necessary to flush even in cache=writethrough mode.
                  */
                 ret = bdrv_driver_pwritev(bs, cluster_offset, pnum,
-                                          &local_qiov, 0);
+                                          &local_qiov,
+                                          BDRV_REQ_WRITE_UNCHANGED);
             }
 
             if (ret < 0) {
@@ -1504,7 +1506,11 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
     assert(!waited || !req->serialising);
     assert(req->overlap_offset <= offset);
     assert(offset + bytes <= req->overlap_offset + req->overlap_bytes);
-    assert(child->perm & BLK_PERM_WRITE);
+    if (flags & BDRV_REQ_WRITE_UNCHANGED) {
+        assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE));
+    } else {
+        assert(child->perm & BLK_PERM_WRITE);
+    }
     assert(end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE);
 
     ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req);
diff --git a/block/mirror.c b/block/mirror.c
index 6aa38db11400..a4197bb975c6 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1134,6 +1134,8 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
         mirror_top_bs->implicit = true;
     }
     mirror_top_bs->total_sectors = bs->total_sectors;
+    mirror_top_bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
+    mirror_top_bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED;
     bdrv_set_aio_context(mirror_top_bs, bdrv_get_aio_context(bs));
 
     /* bdrv_append takes ownership of the mirror_top_bs reference, need to keep
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 6b8b63514afc..2dc23005b7c6 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -1577,9 +1577,9 @@ static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
         case QCOW2_CLUSTER_COMPRESSED:
             /* Compressed clusters don't have QCOW_OFLAG_COPIED */
             if (l2_entry & QCOW_OFLAG_COPIED) {
-                fprintf(stderr, "ERROR: cluster %" PRId64 ": "
+                fprintf(stderr, "ERROR: coffset=0x%" PRIx64 ": "
                     "copied flag must never be set for compressed "
-                    "clusters\n", l2_entry >> s->cluster_bits);
+                    "clusters\n", l2_entry & s->cluster_offset_mask);
                 l2_entry &= ~QCOW_OFLAG_COPIED;
                 res->corruptions++;
             }
diff --git a/block/qcow2.c b/block/qcow2.c
index 2f36e632f93f..6d532470a8e8 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -802,23 +802,30 @@ static void read_cache_sizes(BlockDriverState *bs, QemuOpts *opts,
         } else if (refcount_cache_size_set) {
             *l2_cache_size = combined_cache_size - *refcount_cache_size;
         } else {
-            *refcount_cache_size = combined_cache_size
-                                 / (DEFAULT_L2_REFCOUNT_SIZE_RATIO + 1);
-            *l2_cache_size = combined_cache_size - *refcount_cache_size;
+            uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
+            uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+            uint64_t min_refcount_cache =
+                (uint64_t) MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
+
+            /* Assign as much memory as possible to the L2 cache, and
+             * use the remainder for the refcount cache */
+            if (combined_cache_size >= max_l2_cache + min_refcount_cache) {
+                *l2_cache_size = max_l2_cache;
+                *refcount_cache_size = combined_cache_size - *l2_cache_size;
+            } else {
+                *refcount_cache_size =
+                    MIN(combined_cache_size, min_refcount_cache);
+                *l2_cache_size = combined_cache_size - *refcount_cache_size;
+            }
         }
     } else {
-        if (!l2_cache_size_set && !refcount_cache_size_set) {
+        if (!l2_cache_size_set) {
             *l2_cache_size = MAX(DEFAULT_L2_CACHE_BYTE_SIZE,
                                  (uint64_t)DEFAULT_L2_CACHE_CLUSTERS
                                  * s->cluster_size);
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!l2_cache_size_set) {
-            *l2_cache_size = *refcount_cache_size
-                           * DEFAULT_L2_REFCOUNT_SIZE_RATIO;
-        } else if (!refcount_cache_size_set) {
-            *refcount_cache_size = *l2_cache_size
-                                 / DEFAULT_L2_REFCOUNT_SIZE_RATIO;
+        }
+        if (!refcount_cache_size_set) {
+            *refcount_cache_size = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
         }
     }
 
diff --git a/block/qcow2.h b/block/qcow2.h
index adf5c3950fd3..01b5250415f0 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -77,10 +77,6 @@
 #define DEFAULT_L2_CACHE_CLUSTERS 8 /* clusters */
 #define DEFAULT_L2_CACHE_BYTE_SIZE 1048576 /* bytes */
 
-/* The refblock cache needs only a fourth of the L2 cache size to cover as many
- * clusters */
-#define DEFAULT_L2_REFCOUNT_SIZE_RATIO 4
-
 #define DEFAULT_CLUSTER_SIZE 65536
 
 
diff --git a/block/quorum.c b/block/quorum.c
index a5051da56e35..e448d7e384c0 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -115,6 +115,7 @@ struct QuorumAIOCB {
     /* Request metadata */
     uint64_t offset;
     uint64_t bytes;
+    int flags;
 
     QEMUIOVector *qiov;         /* calling IOV */
 
@@ -157,7 +158,8 @@ static bool quorum_64bits_compare(QuorumVoteValue *a, QuorumVoteValue *b)
 static QuorumAIOCB *quorum_aio_get(BlockDriverState *bs,
                                    QEMUIOVector *qiov,
                                    uint64_t offset,
-                                   uint64_t bytes)
+                                   uint64_t bytes,
+                                   int flags)
 {
     BDRVQuorumState *s = bs->opaque;
     QuorumAIOCB *acb = g_new(QuorumAIOCB, 1);
@@ -168,6 +170,7 @@ static QuorumAIOCB *quorum_aio_get(BlockDriverState *bs,
         .bs                 = bs,
         .offset             = offset,
         .bytes              = bytes,
+        .flags              = flags,
         .qiov               = qiov,
         .votes.compare      = quorum_sha256_compare,
         .votes.vote_list    = QLIST_HEAD_INITIALIZER(acb.votes.vote_list),
@@ -271,9 +274,11 @@ static void quorum_rewrite_entry(void *opaque)
     BDRVQuorumState *s = acb->bs->opaque;
 
     /* Ignore any errors, it's just a correction attempt for already
-     * corrupted data. */
+     * corrupted data.
+     * Mask out BDRV_REQ_WRITE_UNCHANGED because this overwrites the
+     * area with different data from the other children. */
     bdrv_co_pwritev(s->children[co->idx], acb->offset, acb->bytes,
-                    acb->qiov, 0);
+                    acb->qiov, acb->flags & ~BDRV_REQ_WRITE_UNCHANGED);
 
     /* Wake up the caller after the last rewrite */
     acb->rewrite_count--;
@@ -673,7 +678,7 @@ static int quorum_co_preadv(BlockDriverState *bs, uint64_t offset,
                             uint64_t bytes, QEMUIOVector *qiov, int flags)
 {
     BDRVQuorumState *s = bs->opaque;
-    QuorumAIOCB *acb = quorum_aio_get(bs, qiov, offset, bytes);
+    QuorumAIOCB *acb = quorum_aio_get(bs, qiov, offset, bytes, flags);
     int ret;
 
     acb->is_read = true;
@@ -699,7 +704,7 @@ static void write_quorum_entry(void *opaque)
 
     sacb->bs = s->children[i]->bs;
     sacb->ret = bdrv_co_pwritev(s->children[i], acb->offset, acb->bytes,
-                                acb->qiov, 0);
+                                acb->qiov, acb->flags);
     if (sacb->ret == 0) {
         acb->success_count++;
     } else {
@@ -719,7 +724,7 @@ static int quorum_co_pwritev(BlockDriverState *bs, uint64_t offset,
                              uint64_t bytes, QEMUIOVector *qiov, int flags)
 {
     BDRVQuorumState *s = bs->opaque;
-    QuorumAIOCB *acb = quorum_aio_get(bs, qiov, offset, bytes);
+    QuorumAIOCB *acb = quorum_aio_get(bs, qiov, offset, bytes, flags);
     int i, ret;
 
     for (i = 0; i < s->num_children; i++) {
@@ -961,6 +966,8 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     }
     s->next_child_index = s->num_children;
 
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
+
     g_free(opened);
     goto exit;
 
diff --git a/block/raw-format.c b/block/raw-format.c
index a378547c9983..fe33693a2da2 100644
--- a/block/raw-format.c
+++ b/block/raw-format.c
@@ -415,10 +415,11 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     bs->sg = bs->file->bs->sg;
-    bs->supported_write_flags = BDRV_REQ_FUA &
-        bs->file->bs->supported_write_flags;
-    bs->supported_zero_flags = (BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
-        bs->file->bs->supported_zero_flags;
+    bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
+        (BDRV_REQ_FUA & bs->file->bs->supported_write_flags);
+    bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+        ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
+            bs->file->bs->supported_zero_flags);
 
     if (bs->probed && !bdrv_is_read_only(bs)) {
         fprintf(stderr,
diff --git a/block/throttle.c b/block/throttle.c
index 95ed06acd8dd..e298827f95e2 100644
--- a/block/throttle.c
+++ b/block/throttle.c
@@ -81,8 +81,10 @@ static int throttle_open(BlockDriverState *bs, QDict *options,
     if (!bs->file) {
         return -EINVAL;
     }
-    bs->supported_write_flags = bs->file->bs->supported_write_flags;
-    bs->supported_zero_flags = bs->file->bs->supported_zero_flags;
+    bs->supported_write_flags = bs->file->bs->supported_write_flags |
+                                BDRV_REQ_WRITE_UNCHANGED;
+    bs->supported_zero_flags = bs->file->bs->supported_zero_flags |
+                               BDRV_REQ_WRITE_UNCHANGED;
 
     return throttle_configure_tgm(bs, tgm, options, errp);
 }
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index feb711fb6a88..8e1547ded27b 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -400,10 +400,10 @@ L2 table entry:
               62:   0 for standard clusters
                     1 for compressed clusters
 
-              63:   0 for a cluster that is unused or requires COW, 1 if its
-                    refcount is exactly one. This information is only accurate
-                    in L2 tables that are reachable from the active L1
-                    table.
+              63:   0 for clusters that are unused, compressed or require COW.
+                    1 for standard clusters whose refcount is exactly one.
+                    This information is only accurate in L2 tables
+                    that are reachable from the active L1 table.
 
 Standard Cluster Descriptor:
 
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 170191a242bf..8a09a5cc5ffc 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -116,31 +116,30 @@ There are three options available, and all of them take bytes:
 "refcount-cache-size":   maximum size of the refcount block cache
 "cache-size":            maximum size of both caches combined
 
-There are two things that need to be taken into account:
+There are a few things that need to be taken into account:
 
  - Both caches must have a size that is a multiple of the cluster size
    (or the cache entry size: see "Using smaller cache sizes" below).
 
- - If you only set one of the options above, QEMU will automatically
-   adjust the others so that the L2 cache is 4 times bigger than the
-   refcount cache.
+ - The default L2 cache size is 8 clusters or 1MB (whichever is more),
+   and the minimum is 2 clusters (or 2 cache entries, see below).
 
-This means that these options are equivalent:
+ - The default (and minimum) refcount cache size is 4 clusters.
 
-   -drive file=hd.qcow2,l2-cache-size=2097152
-   -drive file=hd.qcow2,refcount-cache-size=524288
-   -drive file=hd.qcow2,cache-size=2621440
+ - If only "cache-size" is specified then QEMU will assign as much
+   memory as possible to the L2 cache before increasing the refcount
+   cache size.
 
-The reason for this 1/4 ratio is to ensure that both caches cover the
-same amount of disk space. Note however that this is only valid with
-the default value of refcount_bits (16). If you are using a different
-value you might want to calculate both cache sizes yourself since QEMU
-will always use the same 1/4 ratio.
+Unlike L2 tables, refcount blocks are not used during normal I/O but
+only during allocations and internal snapshots. In most cases they are
+accessed sequentially (even during random guest I/O) so increasing the
+refcount cache size won't have any measurable effect in performance
+(this can change if you are using internal snapshots, so you may want
+to think about increasing the cache size if you use them heavily).
 
-It's also worth mentioning that there's no strict need for both caches
-to cover the same amount of disk space. The refcount cache is used
-much less often than the L2 cache, so it's perfectly reasonable to
-keep it small.
+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
+L2 cache size. This resulted in unnecessarily large caches, so now the
+refcount cache is as small as possible unless overridden by the user.
 
 
 Using smaller cache entries
diff --git a/include/block/block.h b/include/block/block.h
index cdec3639a353..3894edda9de1 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -54,8 +54,12 @@ typedef enum {
     BDRV_REQ_FUA                = 0x10,
     BDRV_REQ_WRITE_COMPRESSED   = 0x20,
 
+    /* Signifies that this write request will not change the visible disk
+     * content. */
+    BDRV_REQ_WRITE_UNCHANGED    = 0x40,
+
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0x3f,
+    BDRV_REQ_MASK               = 0x7f,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {
@@ -205,6 +209,9 @@ enum {
      * This permission (which is weaker than BLK_PERM_WRITE) is both enough and
      * required for writes to the block node when the caller promises that
      * the visible disk content doesn't change.
+     *
+     * As the BLK_PERM_WRITE permission is strictly stronger, either is
+     * sufficient to perform an unchanging write.
      */
     BLK_PERM_WRITE_UNCHANGED    = 0x04,
 
diff --git a/include/block/block_int.h b/include/block/block_int.h
index e3d6219f4e37..76b589da576a 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -656,10 +656,24 @@ struct BlockDriverState {
     /* I/O Limits */
     BlockLimits bl;
 
-    /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */
+    /* Flags honored during pwrite (so far: BDRV_REQ_FUA,
+     * BDRV_REQ_WRITE_UNCHANGED).
+     * If a driver does not support BDRV_REQ_WRITE_UNCHANGED, those
+     * writes will be issued as normal writes without the flag set.
+     * This is important to note for drivers that do not explicitly
+     * request a WRITE permission for their children and instead take
+     * the same permissions as their parent did (this is commonly what
+     * block filters do).  Such drivers have to be aware that the
+     * parent may have taken a WRITE_UNCHANGED permission only and is
+     * issuing such requests.  Drivers either must make sure that
+     * these requests do not result in plain WRITE accesses (usually
+     * by supporting BDRV_REQ_WRITE_UNCHANGED, and then forwarding
+     * every incoming write request as-is, including potentially that
+     * flag), or they have to explicitly take the WRITE permission for
+     * their children. */
     unsigned int supported_write_flags;
     /* Flags honored during pwrite_zeroes (so far: BDRV_REQ_FUA,
-     * BDRV_REQ_MAY_UNMAP) */
+     * BDRV_REQ_MAY_UNMAP, BDRV_REQ_WRITE_UNCHANGED) */
     unsigned int supported_zero_flags;
 
     /* the following member gives a name to every node on the bs graph. */
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 17ffd44cce3b..55728cb82327 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2510,11 +2510,12 @@
 # @vxhs: Since 2.10
 # @throttle: Since 2.11
 # @nvme: Since 2.12
+# @copy-on-read: Since 2.13
 #
 # Since: 2.9
 ##
 { 'enum': 'BlockdevDriver',
-  'data': [ 'blkdebug', 'blkverify', 'bochs', 'cloop',
+  'data': [ 'blkdebug', 'blkverify', 'bochs', 'cloop', 'copy-on-read',
             'dmg', 'file', 'ftp', 'ftps', 'gluster', 'host_cdrom',
             'host_device', 'http', 'https', 'iscsi', 'luks', 'nbd', 'nfs',
             'null-aio', 'null-co', 'nvme', 'parallels', 'qcow', 'qcow2', 'qed',
@@ -3531,6 +3532,7 @@
       'blkverify':  'BlockdevOptionsBlkverify',
       'bochs':      'BlockdevOptionsGenericFormat',
       'cloop':      'BlockdevOptionsGenericFormat',
+      'copy-on-read':'BlockdevOptionsGenericFormat',
       'dmg':        'BlockdevOptionsGenericFormat',
       'file':       'BlockdevOptionsFile',
       'ftp':        'BlockdevOptionsCurlFtp',
@@ -4058,6 +4060,7 @@
       'blkverify':      'BlockdevCreateNotSupported',
       'bochs':          'BlockdevCreateNotSupported',
       'cloop':          'BlockdevCreateNotSupported',
+      'copy-on-read':   'BlockdevCreateNotSupported',
       'dmg':            'BlockdevCreateNotSupported',
       'file':           'BlockdevCreateOptionsFile',
       'ftp':            'BlockdevCreateNotSupported',
diff --git a/qemu-img.c b/qemu-img.c
index ea62d2d61e85..60e45ec10365 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -277,12 +277,12 @@ static BlockBackend *img_open_opts(const char *optstr,
     options = qemu_opts_to_qdict(opts, NULL);
     if (force_share) {
         if (qdict_haskey(options, BDRV_OPT_FORCE_SHARE)
-            && !qdict_get_bool(options, BDRV_OPT_FORCE_SHARE)) {
+            && strcmp(qdict_get_str(options, BDRV_OPT_FORCE_SHARE), "on")) {
             error_report("--force-share/-U conflicts with image options");
             qobject_unref(options);
             return NULL;
         }
-        qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
+        qdict_put_str(options, BDRV_OPT_FORCE_SHARE, "on");
     }
     blk = blk_new_open(NULL, NULL, options, flags, &local_err);
     if (!blk) {
@@ -3381,7 +3381,7 @@ static int img_resize(int argc, char **argv)
     Error *err = NULL;
     int c, ret, relative;
     const char *filename, *fmt, *size;
-    int64_t n, total_size, current_size;
+    int64_t n, total_size, current_size, new_size;
     bool quiet = false;
     BlockBackend *blk = NULL;
     PreallocMode prealloc = PREALLOC_MODE_OFF;
@@ -3557,11 +3557,42 @@ static int img_resize(int argc, char **argv)
     }
 
     ret = blk_truncate(blk, total_size, prealloc, &err);
-    if (!ret) {
-        qprintf(quiet, "Image resized.\n");
-    } else {
+    if (ret < 0) {
         error_report_err(err);
+        goto out;
+    }
+
+    new_size = blk_getlength(blk);
+    if (new_size < 0) {
+        error_report("Failed to verify truncated image length: %s",
+                     strerror(-new_size));
+        ret = -1;
+        goto out;
     }
+
+    /* Some block drivers implement a truncation method, but only so
+     * the user can cause qemu to refresh the image's size from disk.
+     * The idea is that the user resizes the image outside of qemu and
+     * then invokes block_resize to inform qemu about it.
+     * (This includes iscsi and file-posix for device files.)
+     * Of course, that is not the behavior someone invoking
+     * qemu-img resize would find useful, so we catch that behavior
+     * here and tell the user. */
+    if (new_size != total_size && new_size == current_size) {
+        error_report("Image was not resized; resizing may not be supported "
+                     "for this image");
+        ret = -1;
+        goto out;
+    }
+
+    if (new_size != total_size) {
+        warn_report("Image should have been resized to %" PRIi64
+                    " bytes, but was resized to %" PRIi64 " bytes",
+                    total_size, new_size);
+    }
+
+    qprintf(quiet, "Image resized.\n");
+
 out:
     blk_unref(blk);
     if (ret) {
diff --git a/qemu-io.c b/qemu-io.c
index 72fee0d8b728..73c638ff8b8d 100644
--- a/qemu-io.c
+++ b/qemu-io.c
@@ -95,12 +95,12 @@ static int openfile(char *name, int flags, bool writethrough, bool force_share,
             opts = qdict_new();
         }
         if (qdict_haskey(opts, BDRV_OPT_FORCE_SHARE)
-            && !qdict_get_bool(opts, BDRV_OPT_FORCE_SHARE)) {
+            && strcmp(qdict_get_str(opts, BDRV_OPT_FORCE_SHARE), "on")) {
             error_report("-U conflicts with image options");
             qobject_unref(opts);
             return 1;
         }
-        qdict_put_bool(opts, BDRV_OPT_FORCE_SHARE, true);
+        qdict_put_str(opts, BDRV_OPT_FORCE_SHARE, "on");
     }
     qemuio_blk = blk_new_open(name, NULL, opts, flags, &local_err);
     if (!qemuio_blk) {
diff --git a/tests/qemu-iotests/122 b/tests/qemu-iotests/122
index 6cf4fcb86632..45b359c2ba05 100755
--- a/tests/qemu-iotests/122
+++ b/tests/qemu-iotests/122
@@ -129,53 +129,6 @@ $QEMU_IO -c "read -P 0x44 1023k    1k" "$TEST_IMG" 2>&1 | _filter_qemu_io | _fil
 $QEMU_IO -c "read -P 0    1024k 1022k" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
 
 
-echo
-echo "=== Corrupted size field in compressed cluster descriptor ==="
-echo
-# Create an empty image and fill half of it with compressed data.
-# The L2 entries of the two compressed clusters are located at
-# 0x800000 and 0x800008, their original values are 0x4008000000a00000
-# and 0x4008000000a00802 (5 sectors for compressed data each).
-_make_test_img 8M -o cluster_size=2M
-$QEMU_IO -c "write -c -P 0x11 0 2M" -c "write -c -P 0x11 2M 2M" "$TEST_IMG" \
-         2>&1 | _filter_qemu_io | _filter_testdir
-
-# Reduce size of compressed data to 4 sectors: this corrupts the image.
-poke_file "$TEST_IMG" $((0x800000)) "\x40\x06"
-$QEMU_IO -c "read  -P 0x11 0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-
-# 'qemu-img check' however doesn't see anything wrong because it
-# doesn't try to decompress the data and the refcounts are consistent.
-# TODO: update qemu-img so this can be detected.
-_check_test_img
-
-# Increase size of compressed data to the maximum (8192 sectors).
-# This makes QEMU read more data (8192 sectors instead of 5, host
-# addresses [0xa00000, 0xdfffff]), but the decompression algorithm
-# stops once we have enough to restore the uncompressed cluster, so
-# the rest of the data is ignored.
-poke_file "$TEST_IMG" $((0x800000)) "\x7f\xfe"
-# Do it also for the second compressed cluster (L2 entry at 0x800008).
-# In this case the compressed data would span 3 host clusters
-# (host addresses: [0xa00802, 0xe00801])
-poke_file "$TEST_IMG" $((0x800008)) "\x7f\xfe"
-
-# Here the image is too small so we're asking QEMU to read beyond the
-# end of the image.
-$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-# But if we grow the image we won't be reading beyond its end anymore.
-$QEMU_IO -c "write -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-
-# The refcount data is however wrong because due to the increased size
-# of the compressed data it now reaches the following host clusters.
-# This can be repaired by qemu-img check by increasing the refcount of
-# those clusters.
-# TODO: update qemu-img to correct the compressed cluster size instead.
-_check_test_img -r all
-$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IO -c "read  -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-
 echo
 echo "=== Full allocation with -S 0 ==="
 echo
diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
index a6b7fe007e20..47d8656db84f 100644
--- a/tests/qemu-iotests/122.out
+++ b/tests/qemu-iotests/122.out
@@ -99,39 +99,6 @@ read 1024/1024 bytes at offset 1047552
 read 1046528/1046528 bytes at offset 1048576
 1022 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 
-=== Corrupted size field in compressed cluster descriptor ===
-
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=8388608
-wrote 2097152/2097152 bytes at offset 0
-2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-wrote 2097152/2097152 bytes at offset 2097152
-2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-read failed: Input/output error
-No errors were found on the image.
-read 4194304/4194304 bytes at offset 0
-4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-wrote 4194304/4194304 bytes at offset 4194304
-4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-read 4194304/4194304 bytes at offset 0
-4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-ERROR cluster 6 refcount=1 reference=3
-ERROR cluster 7 refcount=1 reference=2
-Repairing cluster 6 refcount=1 reference=3
-Repairing cluster 7 refcount=1 reference=2
-Repairing OFLAG_COPIED data cluster: l2_entry=8000000000c00000 refcount=3
-Repairing OFLAG_COPIED data cluster: l2_entry=8000000000e00000 refcount=2
-The following inconsistencies were found and repaired:
-
-    0 leaked clusters
-    4 corruptions
-
-Double checking the fixed image now...
-No errors were found on the image.
-read 4194304/4194304 bytes at offset 0
-4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-read 4194304/4194304 bytes at offset 4194304
-4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-
 === Full allocation with -S 0 ===
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
diff --git a/tests/qemu-iotests/137.out b/tests/qemu-iotests/137.out
index e28e1eadbad7..96724a6c3323 100644
--- a/tests/qemu-iotests/137.out
+++ b/tests/qemu-iotests/137.out
@@ -22,7 +22,7 @@ refcount-cache-size may not exceed cache-size
 L2 cache size too big
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
 L2 cache entry size must be a power of two between 512 and the cluster size (65536)
-L2 cache size too big
+Refcount cache size too big
 Conflicting values for qcow2 options 'overlap-check' ('constant') and 'overlap-check.template' ('all')
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
 Unsupported value 'blubb' for qcow2 option 'overlap-check'. Allowed are any of the following: none, constant, cached, all
diff --git a/tests/qemu-iotests/153 b/tests/qemu-iotests/153
index a0fd81548301..ec508c758f55 100755
--- a/tests/qemu-iotests/153
+++ b/tests/qemu-iotests/153
@@ -242,6 +242,23 @@ _run_cmd $QEMU_IO "${TEST_IMG}" -c 'write 0 512'
 
 _cleanup_qemu
 
+echo
+echo "== Detecting -U and force-share conflicts =="
+
+echo
+echo 'No conflict:'
+$QEMU_IMG info -U --image-opts driver=null-co,force-share=on
+echo
+echo 'Conflict:'
+$QEMU_IMG info -U --image-opts driver=null-co,force-share=off
+
+echo
+echo 'No conflict:'
+$QEMU_IO -c 'open -r -U -o driver=null-co,force-share=on'
+echo
+echo 'Conflict:'
+$QEMU_IO -c 'open -r -U -o driver=null-co,force-share=off'
+
 # success, all done
 echo "*** done"
 rm -f $seq.full
diff --git a/tests/qemu-iotests/153.out b/tests/qemu-iotests/153.out
index bb721cb747c8..2510762ba1f0 100644
--- a/tests/qemu-iotests/153.out
+++ b/tests/qemu-iotests/153.out
@@ -399,4 +399,20 @@ Is another process using the image?
 Closing the other
 
 _qemu_io_wrapper TEST_DIR/t.qcow2 -c write 0 512
+
+== Detecting -U and force-share conflicts ==
+
+No conflict:
+image: null-co://
+file format: null-co
+virtual size: 1.0G (1073741824 bytes)
+disk size: unavailable
+
+Conflict:
+qemu-img: --force-share/-U conflicts with image options
+
+No conflict:
+
+Conflict:
+-U conflicts with image options
 *** done
diff --git a/tests/qemu-iotests/181 b/tests/qemu-iotests/181
index 5e767c6195d2..e02979378da4 100755
--- a/tests/qemu-iotests/181
+++ b/tests/qemu-iotests/181
@@ -96,6 +96,19 @@ echo
 # Enable postcopy-ram capability both on source and destination
 silent=yes
 _send_qemu_cmd $dest 'migrate_set_capability postcopy-ram on' "(qemu)"
+
+qemu_error_no_exit=yes success_or_failure=yes \
+    _send_qemu_cmd $dest '' "(qemu)" "Postcopy is not supported"
+if [ ${QEMU_STATUS[$dest]} -lt 0 ]; then
+    _send_qemu_cmd $dest '' "(qemu)"
+
+    _send_qemu_cmd $src 'quit' ""
+    _send_qemu_cmd $dest 'quit' ""
+    wait=1 _cleanup_qemu
+
+    _notrun 'Postcopy is not supported'
+fi
+
 _send_qemu_cmd $src 'migrate_set_speed 4k' "(qemu)"
 _send_qemu_cmd $src 'migrate_set_capability postcopy-ram on' "(qemu)"
 _send_qemu_cmd $src "migrate -d unix:${MIG_SOCKET}" "(qemu)"
diff --git a/tests/qemu-iotests/197 b/tests/qemu-iotests/197
index 5e869fe2b776..3ae4975eecdd 100755
--- a/tests/qemu-iotests/197
+++ b/tests/qemu-iotests/197
@@ -44,6 +44,7 @@ esac
 _cleanup()
 {
     _cleanup_test_img
+    rm -f "$TEST_WRAP"
     rm -f "$BLKDBG_CONF"
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
diff --git a/tests/qemu-iotests/201 b/tests/qemu-iotests/201
index 11f640f5dfc5..c1a1e00077f8 100755
--- a/tests/qemu-iotests/201
+++ b/tests/qemu-iotests/201
@@ -82,6 +82,19 @@ echo
 
 silent=yes
 _send_qemu_cmd $dest 'migrate_set_capability postcopy-ram on' "(qemu)"
+
+qemu_error_no_exit=yes success_or_failure=yes \
+    _send_qemu_cmd $dest '' "(qemu)" "Postcopy is not supported"
+if [ ${QEMU_STATUS[$dest]} -lt 0 ]; then
+    _send_qemu_cmd $dest '' "(qemu)"
+
+    _send_qemu_cmd $src 'quit' ""
+    _send_qemu_cmd $dest 'quit' ""
+    wait=1 _cleanup_qemu
+
+    _notrun 'Postcopy is not supported'
+fi
+
 _send_qemu_cmd $src 'migrate_set_capability postcopy-ram on' "(qemu)"
 _send_qemu_cmd $src "migrate -d unix:${MIG_SOCKET}" "(qemu)"
 
diff --git a/tests/qemu-iotests/214 b/tests/qemu-iotests/214
new file mode 100755
index 000000000000..c46ca2a6dde0
--- /dev/null
+++ b/tests/qemu-iotests/214
@@ -0,0 +1,97 @@
+#!/bin/bash
+#
+# Test qcow2 image compression
+#
+# Copyright (C) 2018 Igalia, S.L.
+# Author: Alberto Garcia <berto@igalia.com>
+#
+# 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/>.
+#
+
+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
+
+# Repairing the corrupted image requires qemu-img check to store a
+# refcount up to 3, which requires at least two refcount bits.
+_unsupported_imgopts 'refcount_bits=1[^0-9]'
+
+
+echo
+echo "=== Corrupted size field in compressed cluster descriptor ==="
+echo
+# Create an empty image and fill half of it with compressed data.
+# The L2 entries of the two compressed clusters are located at
+# 0x800000 and 0x800008, their original values are 0x4008000000a00000
+# and 0x4008000000a00802 (5 sectors for compressed data each).
+_make_test_img 8M -o cluster_size=2M
+$QEMU_IO -c "write -c -P 0x11 0 2M" -c "write -c -P 0x11 2M 2M" "$TEST_IMG" \
+         2>&1 | _filter_qemu_io | _filter_testdir
+
+# Reduce size of compressed data to 4 sectors: this corrupts the image.
+poke_file "$TEST_IMG" $((0x800000)) "\x40\x06"
+$QEMU_IO -c "read  -P 0x11 0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+
+# 'qemu-img check' however doesn't see anything wrong because it
+# doesn't try to decompress the data and the refcounts are consistent.
+# TODO: update qemu-img so this can be detected.
+_check_test_img
+
+# Increase size of compressed data to the maximum (8192 sectors).
+# This makes QEMU read more data (8192 sectors instead of 5, host
+# addresses [0xa00000, 0xdfffff]), but the decompression algorithm
+# stops once we have enough to restore the uncompressed cluster, so
+# the rest of the data is ignored.
+poke_file "$TEST_IMG" $((0x800000)) "\x7f\xfe"
+# Do it also for the second compressed cluster (L2 entry at 0x800008).
+# In this case the compressed data would span 3 host clusters
+# (host addresses: [0xa00802, 0xe00801])
+poke_file "$TEST_IMG" $((0x800008)) "\x7f\xfe"
+
+# Here the image is too small so we're asking QEMU to read beyond the
+# end of the image.
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+# But if we grow the image we won't be reading beyond its end anymore.
+$QEMU_IO -c "write -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+
+# The refcount data is however wrong because due to the increased size
+# of the compressed data it now reaches the following host clusters.
+# This can be repaired by qemu-img check by increasing the refcount of
+# those clusters.
+# TODO: update qemu-img to correct the compressed cluster size instead.
+_check_test_img -r all
+$QEMU_IO -c "read  -P 0x11  0 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+$QEMU_IO -c "read  -P 0x22 4M 4M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
+
+# success, all done
+echo '*** done'
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/214.out b/tests/qemu-iotests/214.out
new file mode 100644
index 000000000000..0fcd8dc051ad
--- /dev/null
+++ b/tests/qemu-iotests/214.out
@@ -0,0 +1,35 @@
+QA output created by 214
+
+=== Corrupted size field in compressed cluster descriptor ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=8388608
+wrote 2097152/2097152 bytes at offset 0
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 2097152
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read failed: Input/output error
+No errors were found on the image.
+read 4194304/4194304 bytes at offset 0
+4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 4194304/4194304 bytes at offset 4194304
+4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 4194304/4194304 bytes at offset 0
+4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+ERROR cluster 6 refcount=1 reference=3
+ERROR cluster 7 refcount=1 reference=2
+Repairing cluster 6 refcount=1 reference=3
+Repairing cluster 7 refcount=1 reference=2
+Repairing OFLAG_COPIED data cluster: l2_entry=8000000000c00000 refcount=3
+Repairing OFLAG_COPIED data cluster: l2_entry=8000000000e00000 refcount=2
+The following inconsistencies were found and repaired:
+
+    0 leaked clusters
+    4 corruptions
+
+Double checking the fixed image now...
+No errors were found on the image.
+read 4194304/4194304 bytes at offset 0
+4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 4194304/4194304 bytes at offset 4194304
+4 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+*** done
diff --git a/tests/qemu-iotests/215 b/tests/qemu-iotests/215
new file mode 100755
index 000000000000..2e616ed659bf
--- /dev/null
+++ b/tests/qemu-iotests/215
@@ -0,0 +1,120 @@
+#!/bin/bash
+#
+# Test case for copy-on-read into qcow2, using the COR filter driver
+#
+# Copyright (C) 2018 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/>.
+#
+
+seq="$(basename $0)"
+echo "QA output created by $seq"
+
+here="$PWD"
+status=1 # failure is the default!
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+TEST_WRAP="$TEST_DIR/t.wrap.qcow2"
+BLKDBG_CONF="$TEST_DIR/blkdebug.conf"
+
+# Sanity check: our use of blkdebug fails if $TEST_DIR contains spaces
+# or other problems
+case "$TEST_DIR" in
+    *[^-_a-zA-Z0-9/]*)
+        _notrun "Suspicious TEST_DIR='$TEST_DIR', cowardly refusing to run" ;;
+esac
+
+_cleanup()
+{
+    _cleanup_test_img
+    rm -f "$TEST_WRAP"
+    rm -f "$BLKDBG_CONF"
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# Test is supported for any backing file; but we force qcow2 for our wrapper.
+_supported_fmt generic
+_supported_proto generic
+_supported_os Linux
+# LUKS support may be possible, but it complicates things.
+_unsupported_fmt luks
+
+echo
+echo '=== Copy-on-read ==='
+echo
+
+# Prep the images
+# VPC rounds image sizes to a specific geometry, force a specific size.
+if [ "$IMGFMT" = "vpc" ]; then
+    IMGOPTS=$(_optstr_add "$IMGOPTS" "force_size")
+fi
+_make_test_img 4G
+$QEMU_IO -c "write -P 55 3G 1k" "$TEST_IMG" | _filter_qemu_io
+IMGPROTO=file IMGFMT=qcow2 IMGOPTS= TEST_IMG_FILE="$TEST_WRAP" \
+    _make_test_img -F "$IMGFMT" -b "$TEST_IMG" | _filter_img_create
+$QEMU_IO -f qcow2 -c "write -z -u 1M 64k" "$TEST_WRAP" | _filter_qemu_io
+
+# Ensure that a read of two clusters, but where one is already allocated,
+# does not re-write the allocated cluster
+cat > "$BLKDBG_CONF" <<EOF
+[inject-error]
+event = "cor_write"
+sector = "2048"
+EOF
+$QEMU_IO -c "open \
+ -o driver=copy-on-read,file.driver=blkdebug,file.config=$BLKDBG_CONF,file.image.driver=qcow2 $TEST_WRAP" \
+ -c "read -P 0 1M 128k" | _filter_qemu_io
+
+# Read the areas we want copied. A zero-length read should still be a
+# no-op.  The next read is under 2G, but aligned so that rounding to
+# clusters copies more than 2G of zeroes. The final read will pick up
+# the non-zero data in the same cluster.  Since a 2G read may exhaust
+# memory on some machines (particularly 32-bit), we skip the test if
+# that fails due to memory pressure.
+$QEMU_IO \
+    -c "open -o driver=copy-on-read,file.driver=qcow2 $TEST_WRAP" \
+    -c "read 0 0" \
+    | _filter_qemu_io
+output=$($QEMU_IO \
+         -c "open -o driver=copy-on-read,file.driver=qcow2 $TEST_WRAP" \
+         -c "read -P 0 1k $((2*1024*1024*1024 - 512))" \
+         2>&1 | _filter_qemu_io)
+case $output in
+    *allocate*)
+        _notrun "Insufficent memory to run test" ;;
+    *) printf '%s\n' "$output" ;;
+esac
+$QEMU_IO \
+    -c "open -o driver=copy-on-read,file.driver=qcow2 $TEST_WRAP" \
+    -c "read -P 0 $((3*1024*1024*1024 + 1024)) 1k" \
+    | _filter_qemu_io
+
+# Copy-on-read is incompatible with read-only
+$QEMU_IO \
+    -c "open -r -o driver=copy-on-read,file.driver=qcow2 $TEST_WRAP" \
+    2>&1 | _filter_testdir
+
+# Break the backing chain, and show that images are identical, and that
+# we properly copied over explicit zeros.
+$QEMU_IMG rebase -u -b "" -f qcow2 "$TEST_WRAP"
+$QEMU_IO -f qcow2 -c map "$TEST_WRAP"
+_check_test_img
+$QEMU_IMG compare -f $IMGFMT -F qcow2 "$TEST_IMG" "$TEST_WRAP"
+
+# success, all done
+echo '*** done'
+status=0
diff --git a/tests/qemu-iotests/215.out b/tests/qemu-iotests/215.out
new file mode 100644
index 000000000000..70b0f5fb1956
--- /dev/null
+++ b/tests/qemu-iotests/215.out
@@ -0,0 +1,26 @@
+QA output created by 215
+
+=== Copy-on-read ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296
+wrote 1024/1024 bytes at offset 3221225472
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Formatting 'TEST_DIR/t.wrap.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
+wrote 65536/65536 bytes at offset 1048576
+64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 131072/131072 bytes at offset 1048576
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 0/0 bytes at offset 0
+0 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 2147483136/2147483136 bytes at offset 1024
+2 GiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1024/1024 bytes at offset 3221226496
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+can't open device TEST_DIR/t.wrap.qcow2: Block node is read-only
+2 GiB (0x80010000) bytes     allocated at offset 0 bytes (0x0)
+1023.938 MiB (0x3fff0000) bytes not allocated at offset 2 GiB (0x80010000)
+64 KiB (0x10000) bytes     allocated at offset 3 GiB (0xc0000000)
+1023.938 MiB (0x3fff0000) bytes not allocated at offset 3 GiB (0xc0010000)
+No errors were found on the image.
+Images are identical.
+*** done
diff --git a/tests/qemu-iotests/216 b/tests/qemu-iotests/216
new file mode 100755
index 000000000000..ca9b47a7fd5b
--- /dev/null
+++ b/tests/qemu-iotests/216
@@ -0,0 +1,115 @@
+#!/usr/bin/env python
+#
+# Copy-on-read tests using a COR filter node
+#
+# Copyright (C) 2018 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: Max Reitz <mreitz@redhat.com>
+
+import iotests
+from iotests import log, qemu_img_pipe, qemu_io, filter_qemu_io
+
+# Need backing file support
+iotests.verify_image_format(supported_fmts=['qcow2', 'qcow', 'qed', 'vmdk'])
+iotests.verify_platform(['linux'])
+
+log('')
+log('=== Copy-on-read across nodes ===')
+log('')
+
+# The old copy-on-read mechanism without a filter node cannot request
+# WRITE_UNCHANGED permissions for its child.  Therefore it just tries
+# to sneak its write by the usual permission system and holds its
+# fingers crossed.  However, that sneaking does not work so well when
+# there is a filter node in the way: That will receive the write
+# request and re-issue a new one to its child, which this time is a
+# proper write request that will make the permission system cough --
+# unless there is someone at the top (like a guest device) that has
+# requested write permissions.
+#
+# A COR filter node, however, can request the proper permissions for
+# its child and therefore is not hit by this issue.
+
+with iotests.FilePath('base.img') as base_img_path, \
+     iotests.FilePath('top.img') as top_img_path, \
+     iotests.VM() as vm:
+
+    log('--- Setting up images ---')
+    log('')
+
+    qemu_img_pipe('create', '-f', iotests.imgfmt, base_img_path, '64M')
+
+    log(filter_qemu_io(qemu_io(base_img_path, '-c', 'write -P 1 0M 1M')))
+
+    qemu_img_pipe('create', '-f', iotests.imgfmt, '-b', base_img_path,
+                  top_img_path)
+
+    log(filter_qemu_io(qemu_io(top_img_path,  '-c', 'write -P 2 1M 1M')))
+
+    log('')
+    log('--- Doing COR ---')
+    log('')
+
+    # Compare with e.g. the following:
+    #   vm.add_drive_raw('if=none,node-name=node0,copy-on-read=on,driver=raw,' \
+    #                    'file.driver=%s,file.file.filename=%s' %
+    #                       (iotests.imgfmt, top_img_path))
+    # (Remove the blockdev-add instead.)
+    # ((Not tested here because it hits an assertion in the permission
+    #   system.))
+
+    vm.launch()
+
+    log(vm.qmp('blockdev-add',
+                    node_name='node0',
+                    driver='copy-on-read',
+                    file={
+                        'driver': 'raw',
+                        'file': {
+                            'driver': 'copy-on-read',
+                            'file': {
+                                'driver': 'raw',
+                                'file': {
+                                    'driver': iotests.imgfmt,
+                                    'file': {
+                                        'driver': 'file',
+                                        'filename': top_img_path
+                                    },
+                                    'backing': {
+                                        'driver': iotests.imgfmt,
+                                        'file': {
+                                            'driver': 'file',
+                                            'filename': base_img_path
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                    }))
+
+    # Trigger COR
+    log(vm.qmp('human-monitor-command',
+               command_line='qemu-io node0 "read 0 64M"'))
+
+    vm.shutdown()
+
+    log('')
+    log('--- Checking COR result ---')
+    log('')
+
+    log(filter_qemu_io(qemu_io(base_img_path, '-c', 'discard 0 64M')))
+    log(filter_qemu_io(qemu_io(top_img_path,  '-c', 'read -P 1 0M 1M')))
+    log(filter_qemu_io(qemu_io(top_img_path,  '-c', 'read -P 2 1M 1M')))
diff --git a/tests/qemu-iotests/216.out b/tests/qemu-iotests/216.out
new file mode 100644
index 000000000000..d3fc590d2966
--- /dev/null
+++ b/tests/qemu-iotests/216.out
@@ -0,0 +1,28 @@
+
+=== Copy-on-read across nodes ===
+
+--- Setting up images ---
+
+wrote 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+wrote 1048576/1048576 bytes at offset 1048576
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+
+--- Doing COR ---
+
+{u'return': {}}
+{u'return': u''}
+
+--- Checking COR result ---
+
+discard 67108864/67108864 bytes at offset 0
+64 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+read 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+read 1048576/1048576 bytes at offset 1048576
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
diff --git a/tests/qemu-iotests/common.qemu b/tests/qemu-iotests/common.qemu
index 85f66b852cd8..f2854849519c 100644
--- a/tests/qemu-iotests/common.qemu
+++ b/tests/qemu-iotests/common.qemu
@@ -52,11 +52,29 @@ _in_fd=4
 # response is not echoed out.
 # If $mismatch_only is set, only non-matching responses will
 # be echoed.
+#
+# If $success_or_failure is set, the meaning of the arguments is
+# changed as follows:
+# $2: A string to search for in the response; if found, this indicates
+#     success and ${QEMU_STATUS[$1]} is set to 0.
+# $3: A string to search for in the response; if found, this indicates
+#     failure and the test is either aborted (if $qemu_error_no_exit
+#     is not set) or ${QEMU_STATUS[$1]} is set to -1 (otherwise).
 function _timed_wait_for()
 {
     local h=${1}
     shift
 
+    if [ -z "${success_or_failure}" ]; then
+        success_match=${*}
+        failure_match=
+    else
+        success_match=${1}
+        failure_match=${2}
+    fi
+
+    timeout=yes
+
     QEMU_STATUS[$h]=0
     while IFS= read -t ${QEMU_COMM_TIMEOUT} resp <&${QEMU_OUT[$h]}
     do
@@ -64,10 +82,18 @@ function _timed_wait_for()
             echo "${resp}" | _filter_testdir | _filter_qemu \
                            | _filter_qemu_io | _filter_qmp | _filter_hmp
         fi
-        grep -q "${*}" < <(echo "${resp}")
+        if [ -n "${failure_match}" ]; then
+            grep -q "${failure_match}" < <(echo "${resp}")
+            if [ $? -eq 0 ]; then
+                timeout=
+                break
+            fi
+        fi
+        grep -q "${success_match}" < <(echo "${resp}")
         if [ $? -eq 0 ]; then
             return
-        elif [ -z "${silent}" ] && [ -n "${mismatch_only}" ]; then
+        fi
+        if [ -z "${silent}" ] && [ -n "${mismatch_only}" ]; then
             echo "${resp}" | _filter_testdir | _filter_qemu \
                            | _filter_qemu_io | _filter_qmp | _filter_hmp
         fi
@@ -75,8 +101,12 @@ function _timed_wait_for()
     done
     QEMU_STATUS[$h]=-1
     if [ -z "${qemu_error_no_exit}" ]; then
-        echo "Timeout waiting for ${*} on handle ${h}"
-        exit 1  # Timeout means the test failed
+        if [ -n "${timeout}" ]; then
+            echo "Timeout waiting for ${success_match} on handle ${h}"
+        else
+            echo "Wrong response matching ${failure_match} on handle ${h}"
+        fi
+        exit 1  # Timeout or wrong match mean the test failed
     fi
 }
 
@@ -96,6 +126,11 @@ function _timed_wait_for()
 # If $qemu_error_no_exit is set, then even if the expected response
 # is not seen, we will not exit.  $QEMU_STATUS[$1] will be set it -1 in
 # that case.
+#
+# If $success_or_failure is set, then the last two strings are the
+# strings the response will be scanned for.  The first of the two
+# indicates success, the latter indicates failure.  Failure is handled
+# like a timeout.
 function _send_qemu_cmd()
 {
     local h=${1}
@@ -109,14 +144,23 @@ function _send_qemu_cmd()
         use_error="no"
     fi
     # This array element extraction is done to accommodate pathnames with spaces
-    cmd=${@: 1:${#@}-1}
-    shift $(($# - 1))
+    if [ -z "${success_or_failure}" ]; then
+        cmd=${@: 1:${#@}-1}
+        shift $(($# - 1))
+    else
+        cmd=${@: 1:${#@}-2}
+        shift $(($# - 2))
+    fi
 
     while [ ${count} -gt 0 ]
     do
         echo "${cmd}" >&${QEMU_IN[${h}]}
         if [ -n "${1}" ]; then
-            qemu_error_no_exit=${use_error} _timed_wait_for ${h} "${1}"
+            if [ -z "${success_or_failure}" ]; then
+                qemu_error_no_exit=${use_error} _timed_wait_for ${h} "${1}"
+            else
+                qemu_error_no_exit=${use_error} _timed_wait_for ${h} "${1}" "${2}"
+            fi
             if [ ${QEMU_STATUS[$h]} -eq 0 ]; then
                 return
             fi
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 5daef2402083..cc8cd8cc8e75 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -212,4 +212,7 @@
 211 rw auto quick
 212 rw auto quick
 213 rw auto quick
+214 rw auto
+215 rw auto quick
+216 rw auto quick
 218 rw auto quick