Merge tag 'pull-nbd-2025-05-14' of https://repo.or.cz/qemu/ericb into staging

NBD patches for 2025-05-14

- Eric Blake: fix blockdev-mirror to no longer inflate sparse destination
  that already reads as zero

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# gpg: Signature made Wed 14 May 2025 22:28:13 EDT
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* tag 'pull-nbd-2025-05-14' of https://repo.or.cz/qemu/ericb:
  mirror: Reduce I/O when destination is detect-zeroes:unmap
  tests: Add iotest mirror-sparse for recent patches
  iotests/common.rc: add disk_usage function
  mirror: Skip writing zeroes when target is already zero
  mirror: Skip pre-zeroing destination if it is already zero
  mirror: Drop redundant zero_target parameter
  mirror: Allow QMP override to declare target already zero
  mirror: Pass full sync mode rather than bool to internals
  mirror: Minor refactoring
  iotests: Improve iotest 194 to mirror data
  block: Add new bdrv_co_is_all_zeroes() function
  block: Let bdrv_co_is_zero_fast consolidate adjacent extents
  file-posix, gluster: Handle zero block status hint better
  block: Expand block status mode from bool to flags

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

22 files changed, 291 insertions, 116 deletions

diff --git a/block/blkdebug.c b/block/blkdebug.c
index 1c1967f8e0..c54aee0c84 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -751,9 +751,9 @@ blkdebug_co_pdiscard(BlockDriverState *bs, int64_t offset, int64_t bytes)
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-blkdebug_co_block_status(BlockDriverState *bs, bool want_zero, int64_t offset,
-                         int64_t bytes, int64_t *pnum, int64_t *map,
-                         BlockDriverState **file)
+blkdebug_co_block_status(BlockDriverState *bs, unsigned int mode,
+                         int64_t offset, int64_t bytes, int64_t *pnum,
+                         int64_t *map, BlockDriverState **file)
 {
     int err;
 
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 00af0b18ac..36d5d3ed9b 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -291,8 +291,8 @@ cbw_co_preadv_snapshot(BlockDriverState *bs, int64_t offset, int64_t bytes,
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-cbw_co_snapshot_block_status(BlockDriverState *bs,
-                             bool want_zero, int64_t offset, int64_t bytes,
+cbw_co_snapshot_block_status(BlockDriverState *bs, unsigned int mode,
+                             int64_t offset, int64_t bytes,
                              int64_t *pnum, int64_t *map,
                              BlockDriverState **file)
 {
diff --git a/block/coroutines.h b/block/coroutines.h
index 79e5efbf75..892646bb7a 100644
--- a/block/coroutines.h
+++ b/block/coroutines.h
@@ -47,7 +47,7 @@ int coroutine_fn GRAPH_RDLOCK
 bdrv_co_common_block_status_above(BlockDriverState *bs,
                                   BlockDriverState *base,
                                   bool include_base,
-                                  bool want_zero,
+                                  unsigned int mode,
                                   int64_t offset,
                                   int64_t bytes,
                                   int64_t *pnum,
@@ -78,7 +78,7 @@ int co_wrapper_mixed_bdrv_rdlock
 bdrv_common_block_status_above(BlockDriverState *bs,
                                BlockDriverState *base,
                                bool include_base,
-                               bool want_zero,
+                               unsigned int mode,
                                int64_t offset,
                                int64_t bytes,
                                int64_t *pnum,
diff --git a/block/file-posix.c b/block/file-posix.c
index ef52ed9169..ec95b74869 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -3273,7 +3273,7 @@ static int find_allocation(BlockDriverState *bs, off_t start,
  * well exceed it.
  */
 static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
-                                            bool want_zero,
+                                            unsigned int mode,
                                             int64_t offset,
                                             int64_t bytes, int64_t *pnum,
                                             int64_t *map,
@@ -3289,7 +3289,8 @@ static int coroutine_fn raw_co_block_status(BlockDriverState *bs,
         return ret;
     }
 
-    if (!want_zero) {
+    if (!(mode & BDRV_WANT_ZERO)) {
+        /* There is no backing file - all bytes are allocated in this file.  */
         *pnum = bytes;
         *map = offset;
         *file = bs;
diff --git a/block/gluster.c b/block/gluster.c
index 8712aa606a..89abd40f31 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -1461,7 +1461,7 @@ exit:
  * (Based on raw_co_block_status() from file-posix.c.)
  */
 static int coroutine_fn qemu_gluster_co_block_status(BlockDriverState *bs,
-                                                     bool want_zero,
+                                                     unsigned int mode,
                                                      int64_t offset,
                                                      int64_t bytes,
                                                      int64_t *pnum,
@@ -1478,7 +1478,7 @@ static int coroutine_fn qemu_gluster_co_block_status(BlockDriverState *bs,
         return ret;
     }
 
-    if (!want_zero) {
+    if (!(mode & BDRV_WANT_ZERO)) {
         *pnum = bytes;
         *map = offset;
         *file = bs;
diff --git a/block/io.c b/block/io.c
index 6d98b0abb9..4fd7768f9c 100644
--- a/block/io.c
+++ b/block/io.c
@@ -38,10 +38,14 @@
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
 #include "system/replay.h"
+#include "qemu/units.h"
 
 /* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
 #define MAX_BOUNCE_BUFFER (32768 << BDRV_SECTOR_BITS)
 
+/* Maximum read size for checking if data reads as zero, in bytes */
+#define MAX_ZERO_CHECK_BUFFER (128 * KiB)
+
 static void coroutine_fn GRAPH_RDLOCK
 bdrv_parent_cb_resize(BlockDriverState *bs);
 
@@ -2364,10 +2368,8 @@ int bdrv_flush_all(void)
  * Drivers not implementing the functionality are assumed to not support
  * backing files, hence all their sectors are reported as allocated.
  *
- * If 'want_zero' is true, the caller is querying for mapping
- * purposes, with a focus on valid BDRV_BLOCK_OFFSET_VALID, _DATA, and
- * _ZERO where possible; otherwise, the result favors larger 'pnum',
- * with a focus on accurate BDRV_BLOCK_ALLOCATED.
+ * 'mode' serves as a hint as to which results are favored; see the
+ * BDRV_WANT_* macros for details.
  *
  * If 'offset' is beyond the end of the disk image the return value is
  * BDRV_BLOCK_EOF and 'pnum' is set to 0.
@@ -2387,7 +2389,7 @@ int bdrv_flush_all(void)
  * set to the host mapping and BDS corresponding to the guest offset.
  */
 static int coroutine_fn GRAPH_RDLOCK
-bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
+bdrv_co_do_block_status(BlockDriverState *bs, unsigned int mode,
                         int64_t offset, int64_t bytes,
                         int64_t *pnum, int64_t *map, BlockDriverState **file)
 {
@@ -2476,7 +2478,7 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
             local_file = bs;
             local_map = aligned_offset;
         } else {
-            ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset,
+            ret = bs->drv->bdrv_co_block_status(bs, mode, aligned_offset,
                                                 aligned_bytes, pnum, &local_map,
                                                 &local_file);
 
@@ -2488,10 +2490,10 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
              * the cache requires an RCU update, so double check here to avoid
              * such an update if possible.
              *
-             * Check want_zero, because we only want to update the cache when we
+             * Check mode, because we only want to update the cache when we
              * have accurate information about what is zero and what is data.
              */
-            if (want_zero &&
+            if (mode == BDRV_WANT_PRECISE &&
                 ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) &&
                 QLIST_EMPTY(&bs->children))
             {
@@ -2548,7 +2550,7 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
 
     if (ret & BDRV_BLOCK_RAW) {
         assert(ret & BDRV_BLOCK_OFFSET_VALID && local_file);
-        ret = bdrv_co_do_block_status(local_file, want_zero, local_map,
+        ret = bdrv_co_do_block_status(local_file, mode, local_map,
                                       *pnum, pnum, &local_map, &local_file);
         goto out;
     }
@@ -2560,7 +2562,7 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
 
         if (!cow_bs) {
             ret |= BDRV_BLOCK_ZERO;
-        } else if (want_zero) {
+        } else if (mode == BDRV_WANT_PRECISE) {
             int64_t size2 = bdrv_co_getlength(cow_bs);
 
             if (size2 >= 0 && offset >= size2) {
@@ -2569,14 +2571,14 @@ bdrv_co_do_block_status(BlockDriverState *bs, bool want_zero,
         }
     }
 
-    if (want_zero && ret & BDRV_BLOCK_RECURSE &&
+    if (mode == BDRV_WANT_PRECISE && ret & BDRV_BLOCK_RECURSE &&
         local_file && local_file != bs &&
         (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
         (ret & BDRV_BLOCK_OFFSET_VALID)) {
         int64_t file_pnum;
         int ret2;
 
-        ret2 = bdrv_co_do_block_status(local_file, want_zero, local_map,
+        ret2 = bdrv_co_do_block_status(local_file, mode, local_map,
                                        *pnum, &file_pnum, NULL, NULL);
         if (ret2 >= 0) {
             /* Ignore errors.  This is just providing extra information, it
@@ -2627,7 +2629,7 @@ int coroutine_fn
 bdrv_co_common_block_status_above(BlockDriverState *bs,
                                   BlockDriverState *base,
                                   bool include_base,
-                                  bool want_zero,
+                                  unsigned int mode,
                                   int64_t offset,
                                   int64_t bytes,
                                   int64_t *pnum,
@@ -2654,7 +2656,7 @@ bdrv_co_common_block_status_above(BlockDriverState *bs,
         return 0;
     }
 
-    ret = bdrv_co_do_block_status(bs, want_zero, offset, bytes, pnum,
+    ret = bdrv_co_do_block_status(bs, mode, offset, bytes, pnum,
                                   map, file);
     ++*depth;
     if (ret < 0 || *pnum == 0 || ret & BDRV_BLOCK_ALLOCATED || bs == base) {
@@ -2671,7 +2673,7 @@ bdrv_co_common_block_status_above(BlockDriverState *bs,
     for (p = bdrv_filter_or_cow_bs(bs); include_base || p != base;
          p = bdrv_filter_or_cow_bs(p))
     {
-        ret = bdrv_co_do_block_status(p, want_zero, offset, bytes, pnum,
+        ret = bdrv_co_do_block_status(p, mode, offset, bytes, pnum,
                                       map, file);
         ++*depth;
         if (ret < 0) {
@@ -2734,7 +2736,8 @@ int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
                                             BlockDriverState **file)
 {
     IO_CODE();
-    return bdrv_co_common_block_status_above(bs, base, false, true, offset,
+    return bdrv_co_common_block_status_above(bs, base, false,
+                                             BDRV_WANT_PRECISE, offset,
                                              bytes, pnum, map, file, NULL);
 }
 
@@ -2752,27 +2755,89 @@ int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, int64_t offset,
  * by @offset and @bytes is known to read as zeroes.
  * Return 1 if that is the case, 0 otherwise and -errno on error.
  * This test is meant to be fast rather than accurate so returning 0
- * does not guarantee non-zero data.
+ * does not guarantee non-zero data; but a return of 1 is reliable.
  */
 int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset,
                                       int64_t bytes)
 {
     int ret;
-    int64_t pnum = bytes;
+    int64_t pnum;
     IO_CODE();
 
-    if (!bytes) {
-        return 1;
+    while (bytes) {
+        ret = bdrv_co_common_block_status_above(bs, NULL, false,
+                                                BDRV_WANT_ZERO, offset, bytes,
+                                                &pnum, NULL, NULL, NULL);
+
+        if (ret < 0) {
+            return ret;
+        }
+        if (!(ret & BDRV_BLOCK_ZERO)) {
+            return 0;
+        }
+        offset += pnum;
+        bytes -= pnum;
     }
 
-    ret = bdrv_co_common_block_status_above(bs, NULL, false, false, offset,
-                                            bytes, &pnum, NULL, NULL, NULL);
+    return 1;
+}
 
+/*
+ * Check @bs (and its backing chain) to see if the entire image is known
+ * to read as zeroes.
+ * Return 1 if that is the case, 0 otherwise and -errno on error.
+ * This test is meant to be fast rather than accurate so returning 0
+ * does not guarantee non-zero data; however, a return of 1 is reliable,
+ * and this function can report 1 in more cases than bdrv_co_is_zero_fast.
+ */
+int coroutine_fn bdrv_co_is_all_zeroes(BlockDriverState *bs)
+{
+    int ret;
+    int64_t pnum, bytes;
+    char *buf;
+    QEMUIOVector local_qiov;
+    IO_CODE();
+
+    bytes = bdrv_co_getlength(bs);
+    if (bytes < 0) {
+        return bytes;
+    }
+
+    /* First probe - see if the entire image reads as zero */
+    ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_WANT_ZERO,
+                                            0, bytes, &pnum, NULL, NULL,
+                                            NULL);
     if (ret < 0) {
         return ret;
     }
+    if (ret & BDRV_BLOCK_ZERO) {
+        return bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+    }
 
-    return (pnum == bytes) && (ret & BDRV_BLOCK_ZERO);
+    /*
+     * Because of the way 'blockdev-create' works, raw files tend to
+     * be created with a non-sparse region at the front to make
+     * alignment probing easier.  If the block starts with only a
+     * small allocated region, it is still worth the effort to see if
+     * the rest of the image is still sparse, coupled with manually
+     * reading the first region to see if it reads zero after all.
+     */
+    if (pnum > MAX_ZERO_CHECK_BUFFER) {
+        return 0;
+    }
+    ret = bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
+    if (ret <= 0) {
+        return ret;
+    }
+    /* Only the head of the image is unknown, and it's small.  Read it.  */
+    buf = qemu_blockalign(bs, pnum);
+    qemu_iovec_init_buf(&local_qiov, buf, pnum);
+    ret = bdrv_driver_preadv(bs, 0, pnum, &local_qiov, 0, 0);
+    if (ret >= 0) {
+        ret = buffer_is_zero(buf, pnum);
+    }
+    qemu_vfree(buf);
+    return ret;
 }
 
 int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
@@ -2782,9 +2847,9 @@ int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
     int64_t dummy;
     IO_CODE();
 
-    ret = bdrv_co_common_block_status_above(bs, bs, true, false, offset,
-                                            bytes, pnum ? pnum : &dummy, NULL,
-                                            NULL, NULL);
+    ret = bdrv_co_common_block_status_above(bs, bs, true, BDRV_WANT_ALLOCATED,
+                                            offset, bytes, pnum ? pnum : &dummy,
+                                            NULL, NULL, NULL);
     if (ret < 0) {
         return ret;
     }
@@ -2817,7 +2882,8 @@ int coroutine_fn bdrv_co_is_allocated_above(BlockDriverState *bs,
     int ret;
     IO_CODE();
 
-    ret = bdrv_co_common_block_status_above(bs, base, include_base, false,
+    ret = bdrv_co_common_block_status_above(bs, base, include_base,
+                                            BDRV_WANT_ALLOCATED,
                                             offset, bytes, pnum, NULL, NULL,
                                             &depth);
     if (ret < 0) {
@@ -3698,8 +3764,8 @@ bdrv_co_preadv_snapshot(BdrvChild *child, int64_t offset, int64_t bytes,
 }
 
 int coroutine_fn
-bdrv_co_snapshot_block_status(BlockDriverState *bs,
-                              bool want_zero, int64_t offset, int64_t bytes,
+bdrv_co_snapshot_block_status(BlockDriverState *bs, unsigned int mode,
+                              int64_t offset, int64_t bytes,
                               int64_t *pnum, int64_t *map,
                               BlockDriverState **file)
 {
@@ -3717,7 +3783,7 @@ bdrv_co_snapshot_block_status(BlockDriverState *bs,
     }
 
     bdrv_inc_in_flight(bs);
-    ret = drv->bdrv_co_snapshot_block_status(bs, want_zero, offset, bytes,
+    ret = drv->bdrv_co_snapshot_block_status(bs, mode, offset, bytes,
                                              pnum, map, file);
     bdrv_dec_in_flight(bs);
 
diff --git a/block/iscsi.c b/block/iscsi.c
index 2f0f4dac09..15b96ee880 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -694,9 +694,9 @@ out_unlock:
 
 
 static int coroutine_fn iscsi_co_block_status(BlockDriverState *bs,
-                                              bool want_zero, int64_t offset,
-                                              int64_t bytes, int64_t *pnum,
-                                              int64_t *map,
+                                              unsigned int mode,
+                                              int64_t offset, int64_t bytes,
+                                              int64_t *pnum, int64_t *map,
                                               BlockDriverState **file)
 {
     IscsiLun *iscsilun = bs->opaque;
diff --git a/block/mirror.c b/block/mirror.c
index a53582f17b..c2c5099c95 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -51,10 +51,10 @@ typedef struct MirrorBlockJob {
     BlockDriverState *to_replace;
     /* Used to block operations on the drive-mirror-replace target */
     Error *replace_blocker;
-    bool is_none_mode;
+    MirrorSyncMode sync_mode;
     BlockMirrorBackingMode backing_mode;
-    /* Whether the target image requires explicit zero-initialization */
-    bool zero_target;
+    /* Whether the target should be assumed to be already zero initialized */
+    bool target_is_zero;
     /*
      * To be accesssed with atomics. Written only under the BQL (required by the
      * current implementation of mirror_change()).
@@ -73,6 +73,7 @@ typedef struct MirrorBlockJob {
     size_t buf_size;
     int64_t bdev_length;
     unsigned long *cow_bitmap;
+    unsigned long *zero_bitmap;
     BdrvDirtyBitmap *dirty_bitmap;
     BdrvDirtyBitmapIter *dbi;
     uint8_t *buf;
@@ -108,9 +109,12 @@ struct MirrorOp {
     int64_t offset;
     uint64_t bytes;
 
-    /* The pointee is set by mirror_co_read(), mirror_co_zero(), and
-     * mirror_co_discard() before yielding for the first time */
+    /*
+     * These pointers are set by mirror_co_read(), mirror_co_zero(), and
+     * mirror_co_discard() before yielding for the first time
+     */
     int64_t *bytes_handled;
+    bool *io_skipped;
 
     bool is_pseudo_op;
     bool is_active_write;
@@ -408,15 +412,34 @@ static void coroutine_fn mirror_co_read(void *opaque)
 static void coroutine_fn mirror_co_zero(void *opaque)
 {
     MirrorOp *op = opaque;
-    int ret;
+    bool write_needed = true;
+    int ret = 0;
 
     op->s->in_flight++;
     op->s->bytes_in_flight += op->bytes;
     *op->bytes_handled = op->bytes;
     op->is_in_flight = true;
 
-    ret = blk_co_pwrite_zeroes(op->s->target, op->offset, op->bytes,
-                               op->s->unmap ? BDRV_REQ_MAY_UNMAP : 0);
+    if (op->s->zero_bitmap) {
+        unsigned long end = DIV_ROUND_UP(op->offset + op->bytes,
+                                         op->s->granularity);
+        assert(QEMU_IS_ALIGNED(op->offset, op->s->granularity));
+        assert(QEMU_IS_ALIGNED(op->bytes, op->s->granularity) ||
+               op->offset + op->bytes == op->s->bdev_length);
+        if (find_next_zero_bit(op->s->zero_bitmap, end,
+                               op->offset / op->s->granularity) == end) {
+            write_needed = false;
+            *op->io_skipped = true;
+        }
+    }
+    if (write_needed) {
+        ret = blk_co_pwrite_zeroes(op->s->target, op->offset, op->bytes,
+                                   op->s->unmap ? BDRV_REQ_MAY_UNMAP : 0);
+    }
+    if (ret >= 0 && op->s->zero_bitmap) {
+        bitmap_set(op->s->zero_bitmap, op->offset / op->s->granularity,
+                   DIV_ROUND_UP(op->bytes, op->s->granularity));
+    }
     mirror_write_complete(op, ret);
 }
 
@@ -435,29 +458,43 @@ static void coroutine_fn mirror_co_discard(void *opaque)
 }
 
 static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
-                               unsigned bytes, MirrorMethod mirror_method)
+                               unsigned bytes, MirrorMethod mirror_method,
+                               bool *io_skipped)
 {
     MirrorOp *op;
     Coroutine *co;
     int64_t bytes_handled = -1;
 
+    assert(QEMU_IS_ALIGNED(offset, s->granularity));
+    assert(QEMU_IS_ALIGNED(bytes, s->granularity) ||
+           offset + bytes == s->bdev_length);
     op = g_new(MirrorOp, 1);
     *op = (MirrorOp){
         .s              = s,
         .offset         = offset,
         .bytes          = bytes,
         .bytes_handled  = &bytes_handled,
+        .io_skipped     = io_skipped,
     };
     qemu_co_queue_init(&op->waiting_requests);
 
     switch (mirror_method) {
     case MIRROR_METHOD_COPY:
+        if (s->zero_bitmap) {
+            bitmap_clear(s->zero_bitmap, offset / s->granularity,
+                         DIV_ROUND_UP(bytes, s->granularity));
+        }
         co = qemu_coroutine_create(mirror_co_read, op);
         break;
     case MIRROR_METHOD_ZERO:
+        /* s->zero_bitmap handled in mirror_co_zero */
         co = qemu_coroutine_create(mirror_co_zero, op);
         break;
     case MIRROR_METHOD_DISCARD:
+        if (s->zero_bitmap) {
+            bitmap_clear(s->zero_bitmap, offset / s->granularity,
+                         DIV_ROUND_UP(bytes, s->granularity));
+        }
         co = qemu_coroutine_create(mirror_co_discard, op);
         break;
     default:
@@ -568,6 +605,7 @@ static void coroutine_fn GRAPH_UNLOCKED mirror_iteration(MirrorBlockJob *s)
         int ret = -1;
         int64_t io_bytes;
         int64_t io_bytes_acct;
+        bool io_skipped = false;
         MirrorMethod mirror_method = MIRROR_METHOD_COPY;
 
         assert(!(offset % s->granularity));
@@ -611,8 +649,10 @@ static void coroutine_fn GRAPH_UNLOCKED mirror_iteration(MirrorBlockJob *s)
         }
 
         io_bytes = mirror_clip_bytes(s, offset, io_bytes);
-        io_bytes = mirror_perform(s, offset, io_bytes, mirror_method);
-        if (mirror_method != MIRROR_METHOD_COPY && write_zeroes_ok) {
+        io_bytes = mirror_perform(s, offset, io_bytes, mirror_method,
+                                  &io_skipped);
+        if (io_skipped ||
+            (mirror_method != MIRROR_METHOD_COPY && write_zeroes_ok)) {
             io_bytes_acct = 0;
         } else {
             io_bytes_acct = io_bytes;
@@ -723,9 +763,10 @@ static int mirror_exit_common(Job *job)
                              &error_abort);
 
     if (!abort && s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
-        BlockDriverState *backing = s->is_none_mode ? src : s->base;
+        BlockDriverState *backing;
         BlockDriverState *unfiltered_target = bdrv_skip_filters(target_bs);
 
+        backing = s->sync_mode == MIRROR_SYNC_MODE_NONE ? src : s->base;
         if (bdrv_cow_bs(unfiltered_target) != backing) {
             bdrv_set_backing_hd(unfiltered_target, backing, &local_err);
             if (local_err) {
@@ -841,15 +882,54 @@ static int coroutine_fn GRAPH_UNLOCKED mirror_dirty_init(MirrorBlockJob *s)
     int64_t offset;
     BlockDriverState *bs;
     BlockDriverState *target_bs = blk_bs(s->target);
-    int ret = -1;
+    int ret = -EIO;
     int64_t count;
+    bool punch_holes =
+        target_bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP &&
+        bdrv_can_write_zeroes_with_unmap(target_bs);
+    int64_t bitmap_length = DIV_ROUND_UP(s->bdev_length, s->granularity);
 
+    /* Determine if the image is already zero, regardless of sync mode.  */
+    s->zero_bitmap = bitmap_new(bitmap_length);
     bdrv_graph_co_rdlock();
     bs = s->mirror_top_bs->backing->bs;
+    if (s->target_is_zero) {
+        ret = 1;
+    } else {
+        ret = bdrv_co_is_all_zeroes(target_bs);
+    }
     bdrv_graph_co_rdunlock();
 
-    if (s->zero_target) {
-        if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
+    /* Determine if a pre-zeroing pass is necessary.  */
+    if (ret < 0) {
+        return ret;
+    } else if (s->sync_mode == MIRROR_SYNC_MODE_TOP) {
+        /*
+         * In TOP mode, there is no benefit to a pre-zeroing pass, but
+         * the zero bitmap can be set if the destination already reads
+         * as zero and we are not punching holes.
+         */
+        if (ret > 0 && !punch_holes) {
+            bitmap_set(s->zero_bitmap, 0, bitmap_length);
+        }
+    } else if (ret == 0 || punch_holes) {
+        /*
+         * Here, we are in FULL mode; our goal is to avoid writing
+         * zeroes if the destination already reads as zero, except
+         * when we are trying to punch holes.  This is possible if
+         * zeroing happened externally (ret > 0) or if we have a fast
+         * way to pre-zero the image (the dirty bitmap will be
+         * populated later by the non-zero portions, the same as for
+         * TOP mode).  If pre-zeroing is not fast, or we need to visit
+         * the entire image in order to punch holes even in the
+         * non-allocated regions of the source, then just mark the
+         * entire image dirty and leave the zero bitmap clear at this
+         * point in time.  Otherwise, it can be faster to pre-zero the
+         * image now, even if we re-write the allocated portions of
+         * the disk later, and the pre-zero pass will populate the
+         * zero bitmap.
+         */
+        if (!bdrv_can_write_zeroes_with_unmap(target_bs) || punch_holes) {
             bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, s->bdev_length);
             return 0;
         }
@@ -858,6 +938,7 @@ static int coroutine_fn GRAPH_UNLOCKED mirror_dirty_init(MirrorBlockJob *s)
         for (offset = 0; offset < s->bdev_length; ) {
             int bytes = MIN(s->bdev_length - offset,
                             QEMU_ALIGN_DOWN(INT_MAX, s->granularity));
+            bool ignored;
 
             mirror_throttle(s);
 
@@ -873,12 +954,15 @@ static int coroutine_fn GRAPH_UNLOCKED mirror_dirty_init(MirrorBlockJob *s)
                 continue;
             }
 
-            mirror_perform(s, offset, bytes, MIRROR_METHOD_ZERO);
+            mirror_perform(s, offset, bytes, MIRROR_METHOD_ZERO, &ignored);
             offset += bytes;
         }
 
         mirror_wait_for_all_io(s);
         s->initial_zeroing_ongoing = false;
+    } else {
+        /* In FULL mode, and image already reads as zero.  */
+        bitmap_set(s->zero_bitmap, 0, bitmap_length);
     }
 
     /* First part, loop on the sectors and initialize the dirty bitmap.  */
@@ -1020,7 +1104,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
     mirror_free_init(s);
 
     s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
-    if (!s->is_none_mode) {
+    if (s->sync_mode != MIRROR_SYNC_MODE_NONE) {
         ret = mirror_dirty_init(s);
         if (ret < 0 || job_is_cancelled(&s->common.job)) {
             goto immediate_exit;
@@ -1163,6 +1247,7 @@ immediate_exit:
     assert(s->in_flight == 0);
     qemu_vfree(s->buf);
     g_free(s->cow_bitmap);
+    g_free(s->zero_bitmap);
     g_free(s->in_flight_bitmap);
     bdrv_dirty_iter_free(s->dbi);
 
@@ -1341,7 +1426,8 @@ do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
 {
     int ret;
     size_t qiov_offset = 0;
-    int64_t bitmap_offset, bitmap_end;
+    int64_t dirty_bitmap_offset, dirty_bitmap_end;
+    int64_t zero_bitmap_offset, zero_bitmap_end;
 
     if (!QEMU_IS_ALIGNED(offset, job->granularity) &&
         bdrv_dirty_bitmap_get(job->dirty_bitmap, offset))
@@ -1385,31 +1471,54 @@ do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
     }
 
     /*
-     * Tails are either clean or shrunk, so for bitmap resetting
-     * we safely align the range down.
+     * Tails are either clean or shrunk, so for dirty bitmap resetting
+     * we safely align the range narrower.  But for zero bitmap, round
+     * range wider for checking or clearing, and narrower for setting.
      */
-    bitmap_offset = QEMU_ALIGN_UP(offset, job->granularity);
-    bitmap_end = QEMU_ALIGN_DOWN(offset + bytes, job->granularity);
-    if (bitmap_offset < bitmap_end) {
-        bdrv_reset_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
-                                bitmap_end - bitmap_offset);
+    dirty_bitmap_offset = QEMU_ALIGN_UP(offset, job->granularity);
+    dirty_bitmap_end = QEMU_ALIGN_DOWN(offset + bytes, job->granularity);
+    if (dirty_bitmap_offset < dirty_bitmap_end) {
+        bdrv_reset_dirty_bitmap(job->dirty_bitmap, dirty_bitmap_offset,
+                                dirty_bitmap_end - dirty_bitmap_offset);
     }
+    zero_bitmap_offset = offset / job->granularity;
+    zero_bitmap_end = DIV_ROUND_UP(offset + bytes, job->granularity);
 
     job_progress_increase_remaining(&job->common.job, bytes);
     job->active_write_bytes_in_flight += bytes;
 
     switch (method) {
     case MIRROR_METHOD_COPY:
+        if (job->zero_bitmap) {
+            bitmap_clear(job->zero_bitmap, zero_bitmap_offset,
+                         zero_bitmap_end - zero_bitmap_offset);
+        }
         ret = blk_co_pwritev_part(job->target, offset, bytes,
                                   qiov, qiov_offset, flags);
         break;
 
     case MIRROR_METHOD_ZERO:
+        if (job->zero_bitmap) {
+            if (find_next_zero_bit(job->zero_bitmap, zero_bitmap_end,
+                                   zero_bitmap_offset) == zero_bitmap_end) {
+                ret = 0;
+                break;
+            }
+        }
         assert(!qiov);
         ret = blk_co_pwrite_zeroes(job->target, offset, bytes, flags);
+        if (job->zero_bitmap && ret >= 0) {
+            bitmap_set(job->zero_bitmap, dirty_bitmap_offset / job->granularity,
+                       (dirty_bitmap_end - dirty_bitmap_offset) /
+                       job->granularity);
+        }
         break;
 
     case MIRROR_METHOD_DISCARD:
+        if (job->zero_bitmap) {
+            bitmap_clear(job->zero_bitmap, zero_bitmap_offset,
+                         zero_bitmap_end - zero_bitmap_offset);
+        }
         assert(!qiov);
         ret = blk_co_pdiscard(job->target, offset, bytes);
         break;
@@ -1430,10 +1539,10 @@ do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
          * at function start, and they must be still dirty, as we've locked
          * the region for in-flight op.
          */
-        bitmap_offset = QEMU_ALIGN_DOWN(offset, job->granularity);
-        bitmap_end = QEMU_ALIGN_UP(offset + bytes, job->granularity);
-        bdrv_set_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
-                              bitmap_end - bitmap_offset);
+        dirty_bitmap_offset = QEMU_ALIGN_DOWN(offset, job->granularity);
+        dirty_bitmap_end = QEMU_ALIGN_UP(offset + bytes, job->granularity);
+        bdrv_set_dirty_bitmap(job->dirty_bitmap, dirty_bitmap_offset,
+                              dirty_bitmap_end - dirty_bitmap_offset);
         qatomic_set(&job->actively_synced, false);
 
         action = mirror_error_action(job, false, -ret);
@@ -1711,15 +1820,16 @@ static BlockJob *mirror_start_job(
                              int creation_flags, BlockDriverState *target,
                              const char *replaces, int64_t speed,
                              uint32_t granularity, int64_t buf_size,
+                             MirrorSyncMode sync_mode,
                              BlockMirrorBackingMode backing_mode,
-                             bool zero_target,
+                             bool target_is_zero,
                              BlockdevOnError on_source_error,
                              BlockdevOnError on_target_error,
                              bool unmap,
                              BlockCompletionFunc *cb,
                              void *opaque,
                              const BlockJobDriver *driver,
-                             bool is_none_mode, BlockDriverState *base,
+                             BlockDriverState *base,
                              bool auto_complete, const char *filter_node_name,
                              bool is_mirror, MirrorCopyMode copy_mode,
                              bool base_ro,
@@ -1878,9 +1988,9 @@ static BlockJob *mirror_start_job(
     s->replaces = g_strdup(replaces);
     s->on_source_error = on_source_error;
     s->on_target_error = on_target_error;
-    s->is_none_mode = is_none_mode;
+    s->sync_mode = sync_mode;
     s->backing_mode = backing_mode;
-    s->zero_target = zero_target;
+    s->target_is_zero = target_is_zero;
     qatomic_set(&s->copy_mode, copy_mode);
     s->base = base;
     s->base_overlay = bdrv_find_overlay(bs, base);
@@ -2009,13 +2119,12 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
                   int creation_flags, int64_t speed,
                   uint32_t granularity, int64_t buf_size,
                   MirrorSyncMode mode, BlockMirrorBackingMode backing_mode,
-                  bool zero_target,
+                  bool target_is_zero,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   bool unmap, const char *filter_node_name,
                   MirrorCopyMode copy_mode, Error **errp)
 {
-    bool is_none_mode;
     BlockDriverState *base;
 
     GLOBAL_STATE_CODE();
@@ -2028,14 +2137,13 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
     }
 
     bdrv_graph_rdlock_main_loop();
-    is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
     base = mode == MIRROR_SYNC_MODE_TOP ? bdrv_backing_chain_next(bs) : NULL;
     bdrv_graph_rdunlock_main_loop();
 
     mirror_start_job(job_id, bs, creation_flags, target, replaces,
-                     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,
+                     speed, granularity, buf_size, mode, backing_mode,
+                     target_is_zero, on_source_error, on_target_error, unmap,
+                     NULL, NULL, &mirror_job_driver, base, false,
                      filter_node_name, true, copy_mode, false, errp);
 }
 
@@ -2061,9 +2169,9 @@ BlockJob *commit_active_start(const char *job_id, BlockDriverState *bs,
 
     job = mirror_start_job(
                      job_id, bs, creation_flags, base, NULL, speed, 0, 0,
-                     MIRROR_LEAVE_BACKING_CHAIN, false,
+                     MIRROR_SYNC_MODE_TOP, MIRROR_LEAVE_BACKING_CHAIN, false,
                      on_error, on_error, true, cb, opaque,
-                     &commit_active_job_driver, false, base, auto_complete,
+                     &commit_active_job_driver, base, auto_complete,
                      filter_node_name, false, MIRROR_COPY_MODE_BACKGROUND,
                      base_read_only, errp);
     if (!job) {
diff --git a/block/nbd.c b/block/nbd.c
index 887841bc81..d5a2b21c6d 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1397,8 +1397,8 @@ nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int64_t bytes)
 }
 
 static int coroutine_fn GRAPH_RDLOCK nbd_client_co_block_status(
-        BlockDriverState *bs, bool want_zero, int64_t offset, int64_t bytes,
-        int64_t *pnum, int64_t *map, BlockDriverState **file)
+        BlockDriverState *bs, unsigned int mode, int64_t offset,
+        int64_t bytes, int64_t *pnum, int64_t *map, BlockDriverState **file)
 {
     int ret, request_ret;
     NBDExtent64 extent = { 0 };
diff --git a/block/null.c b/block/null.c
index dc0b1fdbd9..4e448d593d 100644
--- a/block/null.c
+++ b/block/null.c
@@ -227,9 +227,9 @@ static int null_reopen_prepare(BDRVReopenState *reopen_state,
 }
 
 static int coroutine_fn null_co_block_status(BlockDriverState *bs,
-                                             bool want_zero, int64_t offset,
-                                             int64_t bytes, int64_t *pnum,
-                                             int64_t *map,
+                                             unsigned int mode,
+                                             int64_t offset, int64_t bytes,
+                                             int64_t *pnum, int64_t *map,
                                              BlockDriverState **file)
 {
     BDRVNullState *s = bs->opaque;
diff --git a/block/parallels.c b/block/parallels.c
index 347ca127f3..3a375e2a8a 100644
--- a/block/parallels.c
+++ b/block/parallels.c
@@ -416,9 +416,9 @@ parallels_co_flush_to_os(BlockDriverState *bs)
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-parallels_co_block_status(BlockDriverState *bs, bool want_zero, int64_t offset,
-                          int64_t bytes, int64_t *pnum, int64_t *map,
-                          BlockDriverState **file)
+parallels_co_block_status(BlockDriverState *bs, unsigned int mode,
+                          int64_t offset, int64_t bytes, int64_t *pnum,
+                          int64_t *map, BlockDriverState **file)
 {
     BDRVParallelsState *s = bs->opaque;
     int count;
diff --git a/block/qcow.c b/block/qcow.c
index da8ad4d243..8a3e7591a9 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -530,7 +530,7 @@ get_cluster_offset(BlockDriverState *bs, uint64_t offset, int allocate,
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-qcow_co_block_status(BlockDriverState *bs, bool want_zero,
+qcow_co_block_status(BlockDriverState *bs, unsigned int mode,
                      int64_t offset, int64_t bytes, int64_t *pnum,
                      int64_t *map, BlockDriverState **file)
 {
diff --git a/block/qcow2.c b/block/qcow2.c
index 7774e7f090..66fba89b41 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2141,9 +2141,9 @@ static void qcow2_join_options(QDict *options, QDict *old_options)
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-qcow2_co_block_status(BlockDriverState *bs, bool want_zero, int64_t offset,
-                      int64_t count, int64_t *pnum, int64_t *map,
-                      BlockDriverState **file)
+qcow2_co_block_status(BlockDriverState *bs, unsigned int mode,
+                      int64_t offset, int64_t count, int64_t *pnum,
+                      int64_t *map, BlockDriverState **file)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t host_offset;
diff --git a/block/qed.c b/block/qed.c
index ac24449ffb..4a36fb3929 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -833,9 +833,9 @@ fail:
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-bdrv_qed_co_block_status(BlockDriverState *bs, bool want_zero, int64_t pos,
-                         int64_t bytes, int64_t *pnum, int64_t *map,
-                         BlockDriverState **file)
+bdrv_qed_co_block_status(BlockDriverState *bs, unsigned int mode,
+                         int64_t pos, int64_t bytes, int64_t *pnum,
+                         int64_t *map, BlockDriverState **file)
 {
     BDRVQEDState *s = bs->opaque;
     size_t len = MIN(bytes, SIZE_MAX);
diff --git a/block/quorum.c b/block/quorum.c
index 30747a6df9..ed8ce801ee 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -1226,7 +1226,7 @@ static void quorum_child_perm(BlockDriverState *bs, BdrvChild *c,
  * region contains zeroes, and BDRV_BLOCK_DATA otherwise.
  */
 static int coroutine_fn GRAPH_RDLOCK
-quorum_co_block_status(BlockDriverState *bs, bool want_zero,
+quorum_co_block_status(BlockDriverState *bs, unsigned int mode,
                        int64_t offset, int64_t count,
                        int64_t *pnum, int64_t *map, BlockDriverState **file)
 {
@@ -1238,7 +1238,7 @@ quorum_co_block_status(BlockDriverState *bs, bool want_zero,
     for (i = 0; i < s->num_children; i++) {
         int64_t bytes;
         ret = bdrv_co_common_block_status_above(s->children[i]->bs, NULL, false,
-                                                want_zero, offset, count,
+                                                mode, offset, count,
                                                 &bytes, NULL, NULL, NULL);
         if (ret < 0) {
             quorum_report_bad(QUORUM_OP_TYPE_READ, offset, count,
diff --git a/block/raw-format.c b/block/raw-format.c
index e08526e2ec..df16ac1ea2 100644
--- a/block/raw-format.c
+++ b/block/raw-format.c
@@ -283,8 +283,8 @@ fail:
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-raw_co_block_status(BlockDriverState *bs, bool want_zero, int64_t offset,
-                    int64_t bytes, int64_t *pnum, int64_t *map,
+raw_co_block_status(BlockDriverState *bs, unsigned int mode,
+                    int64_t offset, int64_t bytes, int64_t *pnum, int64_t *map,
                     BlockDriverState **file)
 {
     BDRVRawState *s = bs->opaque;
diff --git a/block/rbd.c b/block/rbd.c
index 7446e66659..951cd63f9a 100644
--- a/block/rbd.c
+++ b/block/rbd.c
@@ -1503,9 +1503,9 @@ static int qemu_rbd_diff_iterate_cb(uint64_t offs, size_t len,
 }
 
 static int coroutine_fn qemu_rbd_co_block_status(BlockDriverState *bs,
-                                                 bool want_zero, int64_t offset,
-                                                 int64_t bytes, int64_t *pnum,
-                                                 int64_t *map,
+                                                 unsigned int mode,
+                                                 int64_t offset, int64_t bytes,
+                                                 int64_t *pnum, int64_t *map,
                                                  BlockDriverState **file)
 {
     BDRVRBDState *s = bs->opaque;
diff --git a/block/snapshot-access.c b/block/snapshot-access.c
index 71ac83c01f..17ed2402db 100644
--- a/block/snapshot-access.c
+++ b/block/snapshot-access.c
@@ -41,11 +41,11 @@ snapshot_access_co_preadv_part(BlockDriverState *bs,
 
 static int coroutine_fn GRAPH_RDLOCK
 snapshot_access_co_block_status(BlockDriverState *bs,
-                                bool want_zero, int64_t offset,
+                                unsigned int mode, int64_t offset,
                                 int64_t bytes, int64_t *pnum,
                                 int64_t *map, BlockDriverState **file)
 {
-    return bdrv_co_snapshot_block_status(bs->file->bs, want_zero, offset,
+    return bdrv_co_snapshot_block_status(bs->file->bs, mode, offset,
                                          bytes, pnum, map, file);
 }
 
diff --git a/block/vdi.c b/block/vdi.c
index a2da6ecab0..3ddc62a569 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -523,8 +523,8 @@ static int vdi_reopen_prepare(BDRVReopenState *state,
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-vdi_co_block_status(BlockDriverState *bs, bool want_zero, int64_t offset,
-                    int64_t bytes, int64_t *pnum, int64_t *map,
+vdi_co_block_status(BlockDriverState *bs, unsigned int mode,
+                    int64_t offset, int64_t bytes, int64_t *pnum, int64_t *map,
                     BlockDriverState **file)
 {
     BDRVVdiState *s = (BDRVVdiState *)bs->opaque;
diff --git a/block/vmdk.c b/block/vmdk.c
index 2adec49912..9c7ab037e1 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -1777,7 +1777,7 @@ static inline uint64_t vmdk_find_offset_in_cluster(VmdkExtent *extent,
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-vmdk_co_block_status(BlockDriverState *bs, bool want_zero,
+vmdk_co_block_status(BlockDriverState *bs, unsigned int mode,
                      int64_t offset, int64_t bytes, int64_t *pnum,
                      int64_t *map, BlockDriverState **file)
 {
diff --git a/block/vpc.c b/block/vpc.c
index 0309e319f6..801ff5793f 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -726,7 +726,7 @@ fail:
 }
 
 static int coroutine_fn GRAPH_RDLOCK
-vpc_co_block_status(BlockDriverState *bs, bool want_zero,
+vpc_co_block_status(BlockDriverState *bs, unsigned int mode,
                     int64_t offset, int64_t bytes,
                     int64_t *pnum, int64_t *map,
                     BlockDriverState **file)
diff --git a/block/vvfat.c b/block/vvfat.c
index 91d69b3cc8..814796d918 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -3134,9 +3134,9 @@ vvfat_co_pwritev(BlockDriverState *bs, int64_t offset, int64_t bytes,
 }
 
 static int coroutine_fn vvfat_co_block_status(BlockDriverState *bs,
-                                              bool want_zero, int64_t offset,
-                                              int64_t bytes, int64_t *n,
-                                              int64_t *map,
+                                              unsigned int mode,
+                                              int64_t offset, int64_t bytes,
+                                              int64_t *n, int64_t *map,
                                               BlockDriverState **file)
 {
     *n = bytes;