Copy-on-Read populates the image file with data read from a backing
image. In order to avoid bloating the image file when all zeroes are
read we should scan the buffer and perform an optimized zero write
operation.
Signed-off-by: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com>
---
block.c | 14 +++++++++++---
1 files changed, 11 insertions(+), 3 deletions(-)
diff --git a/block.c b/block.c
index 5ebbd4d..967a583 100644
--- a/block.c
+++ b/block.c
@@ -1506,6 +1506,7 @@ static int coroutine_fn
bdrv_co_copy_on_readv(BlockDriverState *bs,
*/
void *bounce_buffer;
+ BlockDriver *drv = bs->drv;
struct iovec iov;
QEMUIOVector bounce_qiov;
int64_t cluster_sector_num;
@@ -1526,14 +1527,21 @@ static int coroutine_fn
bdrv_co_copy_on_readv(BlockDriverState *bs,
iov.iov_base = bounce_buffer = qemu_blockalign(bs, iov.iov_len);
qemu_iovec_init_external(&bounce_qiov, &iov, 1);
- ret = bs->drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors,
- &bounce_qiov);
+ ret = drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors,
+ &bounce_qiov);
if (ret < 0) {
goto err;
}
- ret = bs->drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors,
+ if (drv->bdrv_co_write_zeroes &&
+ buffer_is_zero(bounce_buffer, iov.iov_len)) {
+ ret = drv->bdrv_co_write_zeroes(bs, cluster_sector_num,
+ cluster_nb_sectors);
+ } else {
+ ret = drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors,
&bounce_qiov);
+ }
+
if (ret < 0) {
/* It might be okay to ignore write errors for guest requests. If this
* is a deliberate copy-on-read then we don't want to ignore the error.
--
1.7.7.3
