Currently, qemu-img convert writes zeroes when it reads zeroes.
Sometimes it does not because the target is initialized to zeroes
anyway, so we do not need to overwrite (and thus potentially allocate)
it.  This is never the case for targets with backing files, though.  But
even them may have an area that is initialized to zeroes, and that is
the area past the end of the backing file (if that is shorter than the
overlay).

So if the target format's unallocated blocks are zero and there is a gap
between the target's backing file's end and the target's end, we do not
have to explicitly write zeroes there.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 qemu-img.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index 855fa52514..5365afbd13 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1542,7 +1542,9 @@ typedef struct ImgConvertState {
     BlockBackend *target;
     bool has_zero_init;
     bool compressed;
+    bool unallocated_blocks_are_zero;
     bool target_has_backing;
+    int64_t target_backing_sectors; /* negative if unknown */
     bool wr_in_order;
     int min_sparse;
     size_t cluster_sectors;
@@ -1571,12 +1573,23 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
 {
     int64_t src_cur_offset;
     int ret, n, src_cur;
+    bool post_backing_zero = false;
 
     convert_select_part(s, sector_num, &src_cur, &src_cur_offset);
 
     assert(s->total_sectors > sector_num);
     n = MIN(s->total_sectors - sector_num, BDRV_REQUEST_MAX_SECTORS);
 
+    if (s->target_backing_sectors >= 0) {
+        if (sector_num >= s->target_backing_sectors) {
+            post_backing_zero = s->unallocated_blocks_are_zero;
+        } else if (sector_num + n > s->target_backing_sectors) {
+            /* Split requests around target_backing_sectors (because
+             * starting from there, zeros are handled differently) */
+            n = s->target_backing_sectors - sector_num;
+        }
+    }
+
     if (s->sector_next_status <= sector_num) {
         int64_t count = n * BDRV_SECTOR_SIZE;
 
@@ -1598,7 +1611,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
         n = DIV_ROUND_UP(count, BDRV_SECTOR_SIZE);
 
         if (ret & BDRV_BLOCK_ZERO) {
-            s->status = BLK_ZERO;
+            s->status = post_backing_zero ? BLK_BACKING_FILE : BLK_ZERO;
         } else if (ret & BDRV_BLOCK_DATA) {
             s->status = BLK_DATA;
         } else {
@@ -2319,6 +2332,16 @@ static int img_convert(int argc, char **argv)
         }
     }
 
+    if (s.target_has_backing) {
+        /* Errors are treated as "backing length unknown" (which means
+         * s.target_backing_sectors has to be negative, which it will
+         * be automatically).  The backing file length is used only
+         * for optimizations, so such a case is not fatal. */
+        s.target_backing_sectors = bdrv_nb_sectors(out_bs->backing->bs);
+    } else {
+        s.target_backing_sectors = -1;
+    }
+
     ret = bdrv_get_info(out_bs, &bdi);
     if (ret < 0) {
         if (s.compressed) {
@@ -2328,6 +2351,7 @@ static int img_convert(int argc, char **argv)
     } else {
         s.compressed = s.compressed || bdi.needs_compressed_writes;
         s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
+        s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero;
     }
 
     ret = convert_do_copy(&s);
-- 
2.14.3

On 05/01/2018 11:57 AM, Max Reitz wrote:
> Currently, qemu-img convert writes zeroes when it reads zeroes.
> Sometimes it does not because the target is initialized to zeroes
> anyway, so we do not need to overwrite (and thus potentially allocate)
> it.  This is never the case for targets with backing files, though.  But
> even them may have an area that is initialized to zeroes, and that is

s/them/they/

> the area past the end of the backing file (if that is shorter than the
> overlay).
> 
> So if the target format's unallocated blocks are zero and there is a gap
> between the target's backing file's end and the target's end, we do not
> have to explicitly write zeroes there.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   qemu-img.c | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)
> 

Some long variable names, but it aids legibility.  Nice fix.

Reviewed-by: Eric Blake <eblake@redhat.com>

(And here's hoping that Red Hat someday flips the switch so that new 
images avoid compat=0.10 in their downstream builds, as we've had quite 
a few corner cases where we already have optimized speed for compat=1.1 ...)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org