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-rw-r--r--block/backup.c5
-rw-r--r--block/block-copy.c12
-rw-r--r--block/copy-before-write.c39
-rw-r--r--block/copy-before-write.h1
-rw-r--r--block/mirror.c11
-rw-r--r--block/replication.c4
-rw-r--r--blockdev.c2
-rw-r--r--include/block/block-common.h2
-rw-r--r--include/block/block-copy.h2
-rw-r--r--include/block/block_int-global-state.h2
-rw-r--r--qapi/block-core.json4
-rw-r--r--tests/qemu-iotests/257.out112
-rw-r--r--tests/qemu-iotests/pylintrc2
-rwxr-xr-xtests/qemu-iotests/tests/backup-discard-source152
-rw-r--r--tests/qemu-iotests/tests/backup-discard-source.out5
15 files changed, 282 insertions, 73 deletions
diff --git a/block/backup.c b/block/backup.c
index ec29d6b810..3dd2e229d2 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -356,7 +356,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
                   BitmapSyncMode bitmap_mode,
-                  bool compress,
+                  bool compress, bool discard_source,
                   const char *filter_node_name,
                   BackupPerf *perf,
                   BlockdevOnError on_source_error,
@@ -457,7 +457,8 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         goto error;
     }
 
-    cbw = bdrv_cbw_append(bs, target, filter_node_name, &bcs, errp);
+    cbw = bdrv_cbw_append(bs, target, filter_node_name, discard_source,
+                          &bcs, errp);
     if (!cbw) {
         goto error;
     }
diff --git a/block/block-copy.c b/block/block-copy.c
index 9ee3dd7ef5..7e3b378528 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -137,6 +137,7 @@ typedef struct BlockCopyState {
     CoMutex lock;
     int64_t in_flight_bytes;
     BlockCopyMethod method;
+    bool discard_source;
     BlockReqList reqs;
     QLIST_HEAD(, BlockCopyCallState) calls;
     /*
@@ -351,7 +352,9 @@ static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
 }
 
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
+                                     BlockDriverState *copy_bitmap_bs,
                                      const BdrvDirtyBitmap *bitmap,
+                                     bool discard_source,
                                      Error **errp)
 {
     ERRP_GUARD();
@@ -367,7 +370,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
         return NULL;
     }
 
-    copy_bitmap = bdrv_create_dirty_bitmap(source->bs, cluster_size, NULL,
+    copy_bitmap = bdrv_create_dirty_bitmap(copy_bitmap_bs, cluster_size, NULL,
                                            errp);
     if (!copy_bitmap) {
         return NULL;
@@ -417,6 +420,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
                                     cluster_size),
     };
 
+    s->discard_source = discard_source;
     block_copy_set_copy_opts(s, false, false);
 
     ratelimit_init(&s->rate_limit);
@@ -588,6 +592,12 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)
     co_put_to_shres(s->mem, t->req.bytes);
     block_copy_task_end(t, ret);
 
+    if (s->discard_source && ret == 0) {
+        int64_t nbytes =
+            MIN(t->req.offset + t->req.bytes, s->len) - t->req.offset;
+        bdrv_co_pdiscard(s->source, t->req.offset, nbytes);
+    }
+
     return ret;
 }
 
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 8aba27a71d..cd65524e26 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -44,6 +44,7 @@ typedef struct BDRVCopyBeforeWriteState {
     BdrvChild *target;
     OnCbwError on_cbw_error;
     uint32_t cbw_timeout_ns;
+    bool discard_source;
 
     /*
      * @lock: protects access to @access_bitmap, @done_bitmap and
@@ -325,14 +326,24 @@ static int coroutine_fn GRAPH_RDLOCK
 cbw_co_pdiscard_snapshot(BlockDriverState *bs, int64_t offset, int64_t bytes)
 {
     BDRVCopyBeforeWriteState *s = bs->opaque;
+    uint32_t cluster_size = block_copy_cluster_size(s->bcs);
+    int64_t aligned_offset = QEMU_ALIGN_UP(offset, cluster_size);
+    int64_t aligned_end = QEMU_ALIGN_DOWN(offset + bytes, cluster_size);
+    int64_t aligned_bytes;
+
+    if (aligned_end <= aligned_offset) {
+        return 0;
+    }
+    aligned_bytes = aligned_end - aligned_offset;
 
     WITH_QEMU_LOCK_GUARD(&s->lock) {
-        bdrv_reset_dirty_bitmap(s->access_bitmap, offset, bytes);
+        bdrv_reset_dirty_bitmap(s->access_bitmap, aligned_offset,
+                                aligned_bytes);
     }
 
-    block_copy_reset(s->bcs, offset, bytes);
+    block_copy_reset(s->bcs, aligned_offset, aligned_bytes);
 
-    return bdrv_co_pdiscard(s->target, offset, bytes);
+    return bdrv_co_pdiscard(s->target, aligned_offset, aligned_bytes);
 }
 
 static void GRAPH_RDLOCK cbw_refresh_filename(BlockDriverState *bs)
@@ -347,6 +358,8 @@ cbw_child_perm(BlockDriverState *bs, BdrvChild *c, BdrvChildRole role,
                uint64_t perm, uint64_t shared,
                uint64_t *nperm, uint64_t *nshared)
 {
+    BDRVCopyBeforeWriteState *s = bs->opaque;
+
     if (!(role & BDRV_CHILD_FILTERED)) {
         /*
          * Target child
@@ -364,9 +377,17 @@ cbw_child_perm(BlockDriverState *bs, BdrvChild *c, BdrvChildRole role,
                            perm, shared, nperm, nshared);
 
         if (!QLIST_EMPTY(&bs->parents)) {
-            if (perm & BLK_PERM_WRITE) {
-                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+            /*
+             * Note, that source child may be shared with backup job. Backup job
+             * does create own blk parent on copy-before-write node, so this
+             * works even if source node does not have any parents before backup
+             * start
+             */
+            *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
+            if (s->discard_source) {
+                *nperm = *nperm | BLK_PERM_WRITE;
             }
+
             *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
         }
     }
@@ -454,7 +475,9 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
             ((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
              bs->file->bs->supported_zero_flags);
 
-    s->bcs = block_copy_state_new(bs->file, s->target, bitmap, errp);
+    s->discard_source = flags & BDRV_O_CBW_DISCARD_SOURCE;
+    s->bcs = block_copy_state_new(bs->file, s->target, bs, bitmap,
+                                  flags & BDRV_O_CBW_DISCARD_SOURCE, errp);
     if (!s->bcs) {
         error_prepend(errp, "Cannot create block-copy-state: ");
         return -EINVAL;
@@ -521,12 +544,14 @@ static BlockDriver bdrv_cbw_filter = {
 BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
+                                  bool discard_source,
                                   BlockCopyState **bcs,
                                   Error **errp)
 {
     BDRVCopyBeforeWriteState *state;
     BlockDriverState *top;
     QDict *opts;
+    int flags = BDRV_O_RDWR | (discard_source ? BDRV_O_CBW_DISCARD_SOURCE : 0);
 
     assert(source->total_sectors == target->total_sectors);
     GLOBAL_STATE_CODE();
@@ -539,7 +564,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     qdict_put_str(opts, "file", bdrv_get_node_name(source));
     qdict_put_str(opts, "target", bdrv_get_node_name(target));
 
-    top = bdrv_insert_node(source, opts, BDRV_O_RDWR, errp);
+    top = bdrv_insert_node(source, opts, flags, errp);
     if (!top) {
         return NULL;
     }
diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index 6e72bb25e9..01af0cd3c4 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -39,6 +39,7 @@
 BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
+                                  bool discard_source,
                                   BlockCopyState **bcs,
                                   Error **errp);
 void bdrv_cbw_drop(BlockDriverState *bs);
diff --git a/block/mirror.c b/block/mirror.c
index 1bdce3b657..61f0a717b7 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -93,6 +93,7 @@ typedef struct MirrorBlockJob {
     int64_t active_write_bytes_in_flight;
     bool prepared;
     bool in_drain;
+    bool base_ro;
 } MirrorBlockJob;
 
 typedef struct MirrorBDSOpaque {
@@ -794,6 +795,10 @@ static int mirror_exit_common(Job *job)
     bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
     bdrv_graph_wrunlock();
 
+    if (abort && s->base_ro && !bdrv_is_read_only(target_bs)) {
+        bdrv_reopen_set_read_only(target_bs, true, NULL);
+    }
+
     bdrv_drained_end(target_bs);
     bdrv_unref(target_bs);
 
@@ -1717,6 +1722,7 @@ static BlockJob *mirror_start_job(
                              bool is_none_mode, BlockDriverState *base,
                              bool auto_complete, const char *filter_node_name,
                              bool is_mirror, MirrorCopyMode copy_mode,
+                             bool base_ro,
                              Error **errp)
 {
     MirrorBlockJob *s;
@@ -1800,6 +1806,7 @@ static BlockJob *mirror_start_job(
     bdrv_unref(mirror_top_bs);
 
     s->mirror_top_bs = mirror_top_bs;
+    s->base_ro = base_ro;
 
     /* No resize for the target either; while the mirror is still running, a
      * consistent read isn't necessarily possible. We could possibly allow
@@ -2029,7 +2036,7 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
                      speed, granularity, buf_size, backing_mode, zero_target,
                      on_source_error, on_target_error, unmap, NULL, NULL,
                      &mirror_job_driver, is_none_mode, base, false,
-                     filter_node_name, true, copy_mode, errp);
+                     filter_node_name, true, copy_mode, false, errp);
 }
 
 BlockJob *commit_active_start(const char *job_id, BlockDriverState *bs,
@@ -2058,7 +2065,7 @@ BlockJob *commit_active_start(const char *job_id, BlockDriverState *bs,
                      on_error, on_error, true, cb, opaque,
                      &commit_active_job_driver, false, base, auto_complete,
                      filter_node_name, false, MIRROR_COPY_MODE_BACKGROUND,
-                     errp);
+                     base_read_only, errp);
     if (!job) {
         goto error_restore_flags;
     }
diff --git a/block/replication.c b/block/replication.c
index ca6bd0a720..0415a5e8b7 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -582,8 +582,8 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
 
         s->backup_job = backup_job_create(
                                 NULL, s->secondary_disk->bs, s->hidden_disk->bs,
-                                0, MIRROR_SYNC_MODE_NONE, NULL, 0, false, NULL,
-                                &perf,
+                                0, MIRROR_SYNC_MODE_NONE, NULL, 0, false, false,
+                                NULL, &perf,
                                 BLOCKDEV_ON_ERROR_REPORT,
                                 BLOCKDEV_ON_ERROR_REPORT, JOB_INTERNAL,
                                 backup_job_completed, bs, NULL, &local_err);
diff --git a/blockdev.c b/blockdev.c
index 528db3452f..835064ed03 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2728,7 +2728,7 @@ static BlockJob *do_backup_common(BackupCommon *backup,
 
     job = backup_job_create(backup->job_id, bs, target_bs, backup->speed,
                             backup->sync, bmap, backup->bitmap_mode,
-                            backup->compress,
+                            backup->compress, backup->discard_source,
                             backup->filter_node_name,
                             &perf,
                             backup->on_source_error,
diff --git a/include/block/block-common.h b/include/block/block-common.h
index a846023a09..338fe5ff7a 100644
--- a/include/block/block-common.h
+++ b/include/block/block-common.h
@@ -243,6 +243,8 @@ typedef enum {
                                       read-write fails */
 #define BDRV_O_IO_URING    0x40000 /* use io_uring instead of the thread pool */
 
+#define BDRV_O_CBW_DISCARD_SOURCE 0x80000 /* for copy-before-write filter */
+
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_NO_FLUSH)
 
 
diff --git a/include/block/block-copy.h b/include/block/block-copy.h
index 0700953ab8..bdc703bacd 100644
--- a/include/block/block-copy.h
+++ b/include/block/block-copy.h
@@ -25,7 +25,9 @@ typedef struct BlockCopyState BlockCopyState;
 typedef struct BlockCopyCallState BlockCopyCallState;
 
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
+                                     BlockDriverState *copy_bitmap_bs,
                                      const BdrvDirtyBitmap *bitmap,
+                                     bool discard_source,
                                      Error **errp);
 
 /* Function should be called prior any actual copy request */
diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
index d2201e27f4..eb2d92a226 100644
--- a/include/block/block_int-global-state.h
+++ b/include/block/block_int-global-state.h
@@ -193,7 +193,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                             MirrorSyncMode sync_mode,
                             BdrvDirtyBitmap *sync_bitmap,
                             BitmapSyncMode bitmap_mode,
-                            bool compress,
+                            bool compress, bool discard_source,
                             const char *filter_node_name,
                             BackupPerf *perf,
                             BlockdevOnError on_source_error,
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 746d1694c2..df5e07debd 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1610,6 +1610,9 @@
 #     node specified by @drive.  If this option is not given, a node
 #     name is autogenerated.  (Since: 4.2)
 #
+# @discard-source: Discard blocks on source which have already been
+#     copied to the target.  (Since 9.1)
+#
 # @x-perf: Performance options.  (Since 6.0)
 #
 # Features:
@@ -1631,6 +1634,7 @@
             '*on-target-error': 'BlockdevOnError',
             '*auto-finalize': 'bool', '*auto-dismiss': 'bool',
             '*filter-node-name': 'str',
+            '*discard-source': 'bool',
             '*x-perf': { 'type': 'BackupPerf',
                          'features': [ 'unstable' ] } } }
 
diff --git a/tests/qemu-iotests/257.out b/tests/qemu-iotests/257.out
index aa76131ca9..c33dd7f3a9 100644
--- a/tests/qemu-iotests/257.out
+++ b/tests/qemu-iotests/257.out
@@ -120,16 +120,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -596,16 +596,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -865,16 +865,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -1341,16 +1341,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -1610,16 +1610,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -2086,16 +2086,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -2355,16 +2355,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -2831,16 +2831,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -3100,16 +3100,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -3576,16 +3576,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -3845,16 +3845,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -4321,16 +4321,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -4590,16 +4590,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
@@ -5066,16 +5066,16 @@ write -P0x67 0x3fe0000 0x20000
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      }
-    ],
-    "drive0": [
+      },
       {
         "busy": false,
         "count": 0,
         "granularity": 65536,
         "persistent": false,
         "recording": false
-      },
+      }
+    ],
+    "drive0": [
       {
         "busy": false,
         "count": 458752,
diff --git a/tests/qemu-iotests/pylintrc b/tests/qemu-iotests/pylintrc
index de2e0c2781..05b75ee59b 100644
--- a/tests/qemu-iotests/pylintrc
+++ b/tests/qemu-iotests/pylintrc
@@ -55,4 +55,4 @@ max-line-length=79
 
 [SIMILARITIES]
 
-min-similarity-lines=6
+min-similarity-lines=10
diff --git a/tests/qemu-iotests/tests/backup-discard-source b/tests/qemu-iotests/tests/backup-discard-source
new file mode 100755
index 0000000000..2391b12acd
--- /dev/null
+++ b/tests/qemu-iotests/tests/backup-discard-source
@@ -0,0 +1,152 @@
+#!/usr/bin/env python3
+#
+# Test backup discard-source parameter
+#
+# Copyright (c) Virtuozzo International GmbH.
+# Copyright (c) Yandex
+#
+# 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/>.
+#
+
+import os
+
+import iotests
+from iotests import qemu_img_create, qemu_img_map, qemu_io
+
+
+temp_img = os.path.join(iotests.test_dir, 'temp')
+source_img = os.path.join(iotests.test_dir, 'source')
+target_img = os.path.join(iotests.test_dir, 'target')
+size = '1M'
+
+
+def get_actual_size(vm, node_name):
+    nodes = vm.cmd('query-named-block-nodes', flat=True)
+    node = next(n for n in nodes if n['node-name'] == node_name)
+    return node['image']['actual-size']
+
+
+class TestBackup(iotests.QMPTestCase):
+    def setUp(self):
+        qemu_img_create('-f', iotests.imgfmt, source_img, size)
+        qemu_img_create('-f', iotests.imgfmt, temp_img, size)
+        qemu_img_create('-f', iotests.imgfmt, target_img, size)
+        qemu_io('-c', 'write 0 1M', source_img)
+
+        self.vm = iotests.VM()
+        self.vm.launch()
+
+        self.vm.cmd('blockdev-add', {
+            'node-name': 'cbw',
+            'driver': 'copy-before-write',
+            'file': {
+                'driver': iotests.imgfmt,
+                'file': {
+                    'driver': 'file',
+                    'filename': source_img,
+                }
+            },
+            'target': {
+                'driver': iotests.imgfmt,
+                'discard': 'unmap',
+                'node-name': 'temp',
+                'file': {
+                    'driver': 'file',
+                    'filename': temp_img
+                }
+            }
+        })
+
+        self.vm.cmd('blockdev-add', {
+            'node-name': 'access',
+            'discard': 'unmap',
+            'driver': 'snapshot-access',
+            'file': 'cbw'
+        })
+
+        self.vm.cmd('blockdev-add', {
+            'driver': iotests.imgfmt,
+            'node-name': 'target',
+            'file': {
+                'driver': 'file',
+                'filename': target_img
+            }
+        })
+
+        self.assertLess(get_actual_size(self.vm, 'temp'), 512 * 1024)
+
+    def tearDown(self):
+        # That should fail, because region is discarded
+        self.vm.hmp_qemu_io('access', 'read 0 1M')
+
+        self.vm.shutdown()
+
+        self.assertTrue('read failed: Permission denied' in self.vm.get_log())
+
+        # Final check that temp image is empty
+        mapping = qemu_img_map(temp_img)
+        self.assertEqual(len(mapping), 1)
+        self.assertEqual(mapping[0]['start'], 0)
+        self.assertEqual(mapping[0]['length'], 1024 * 1024)
+        self.assertEqual(mapping[0]['data'], False)
+
+        os.remove(temp_img)
+        os.remove(source_img)
+        os.remove(target_img)
+
+    def do_backup(self):
+        self.vm.cmd('blockdev-backup', device='access',
+                    sync='full', target='target',
+                    job_id='backup0',
+                    discard_source=True)
+
+        self.vm.event_wait(name='BLOCK_JOB_COMPLETED')
+
+    def test_discard_written(self):
+        """
+        1. Guest writes
+        2. copy-before-write operation, data is stored to temp
+        3. start backup(discard_source=True), check that data is
+           removed from temp
+        """
+        # Trigger copy-before-write operation
+        result = self.vm.hmp_qemu_io('cbw', 'write 0 1M')
+        self.assert_qmp(result, 'return', '')
+
+        # Check that data is written to temporary image
+        self.assertGreater(get_actual_size(self.vm, 'temp'), 1024 * 1024)
+
+        self.do_backup()
+
+    def test_discard_cbw(self):
+        """
+        1. do backup(discard_source=True), which should inform
+           copy-before-write that data is not needed anymore
+        2. Guest writes
+        3. Check that copy-before-write operation is not done
+        """
+        self.do_backup()
+
+        # Try trigger copy-before-write operation
+        result = self.vm.hmp_qemu_io('cbw', 'write 0 1M')
+        self.assert_qmp(result, 'return', '')
+
+        # Check that data is not written to temporary image, as region
+        # is discarded from copy-before-write process
+        self.assertLess(get_actual_size(self.vm, 'temp'), 512 * 1024)
+
+
+if __name__ == '__main__':
+    iotests.main(supported_fmts=['qcow2'],
+                 supported_protocols=['file'])
diff --git a/tests/qemu-iotests/tests/backup-discard-source.out b/tests/qemu-iotests/tests/backup-discard-source.out
new file mode 100644
index 0000000000..fbc63e62f8
--- /dev/null
+++ b/tests/qemu-iotests/tests/backup-discard-source.out
@@ -0,0 +1,5 @@
+..
+----------------------------------------------------------------------
+Ran 2 tests
+
+OK