[Qemu-block] [PATCH 2/3] qemu-img: Use zero writes after source backing EOF

[Max Reitz]

Past the end of the source backing file, we memset() buf_old to zero, so
it is clearly easy to use blk_pwrite_zeroes() instead of blk_pwrite()
then.

Signed-off-by: Max Reitz <address@hidden>
---
 qemu-img.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index dd684d8bf0..2552e7dad6 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3403,6 +3403,8 @@ static int img_rebase(int argc, char **argv)
         }
 
         for (offset = 0; offset < size; offset += n) {
+            bool buf_old_is_zero = false;
+
             /* How many bytes can we handle with the next read? */
             n = MIN(IO_BUF_SIZE, size - offset);
 
@@ -3423,6 +3425,7 @@ static int img_rebase(int argc, char **argv)
              */
             if (offset >= old_backing_size) {
                 memset(buf_old, 0, n);
+                buf_old_is_zero = true;
             } else {
                 if (offset + n > old_backing_size) {
                     n = old_backing_size - offset;
@@ -3458,8 +3461,12 @@ static int img_rebase(int argc, char **argv)
                 if (compare_buffers(buf_old + written, buf_new + written,
                                     n - written, &pnum))
                 {
-                    ret = blk_pwrite(blk, offset + written,
-                                     buf_old + written, pnum, 0);
+                    if (buf_old_is_zero) {
+                        ret = blk_pwrite_zeroes(blk, offset + written, pnum, 
0);
+                    } else {
+                        ret = blk_pwrite(blk, offset + written,
+                                         buf_old + written, pnum, 0);
+                    }
                     if (ret < 0) {
                         error_report("Error while writing to COW image: %s",
                             strerror(-ret));
-- 
2.17.1