vmdk: fix comment for vmdk_co_write_zeroes
The comment was truncated. Add the missing parts, especially explain why
we need zero_dry_run.
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/block/vmdk.c b/block/vmdk.c
index 3756333..e6c50b1 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -1200,8 +1200,10 @@
/**
* vmdk_write:
* @zeroed: buf is ignored (data is zero), use zeroed_grain GTE feature
- * if possible, otherwise return -ENOTSUP.
- * @zero_dry_run: used for zeroed == true only, don't update L2 table, just
+ * if possible, otherwise return -ENOTSUP.
+ * @zero_dry_run: used for zeroed == true only, don't update L2 table, just try
+ * with each cluster. By dry run we can find if the zero write
+ * is possible without modifying image data.
*
* Returns: error code with 0 for success.
*/
@@ -1328,6 +1330,8 @@
int ret;
BDRVVmdkState *s = bs->opaque;
qemu_co_mutex_lock(&s->lock);
+ /* write zeroes could fail if sectors not aligned to cluster, test it with
+ * dry_run == true before really updating image */
ret = vmdk_write(bs, sector_num, NULL, nb_sectors, true, true);
if (!ret) {
ret = vmdk_write(bs, sector_num, NULL, nb_sectors, true, false);
