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qemu-img: Use zero writes after source backing EOF

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 <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
tags/v4.1.0-rc0
Max Reitz 4 months ago
parent
commit
1c6e877992
1 changed files with 9 additions and 2 deletions
  1. 9
    2
      qemu-img.c

+ 9
- 2
qemu-img.c View File

@@ -3432,6 +3432,8 @@ 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);

@@ -3452,6 +3454,7 @@ 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;
@@ -3487,8 +3490,12 @@ 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));

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