People get surprised when, after "qemu-imc create -f raw /dev/sdX", they
still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
header. While this is natural because raw doesn't need to write any
magic bytes during creation, hdev_create is free to clear out the first
sector to make sure the stale qcow2 header doesn't cause such a
confusion.

Signed-off-by: Fam Zheng <f...@redhat.com>
---
 block/file-posix.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/file-posix.c b/block/file-posix.c
index f4de022ae0..1d8ef6f873 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2703,6 +2703,17 @@ static int hdev_create(const char *filename, QemuOpts 
*opts,
         ret = -ENOSPC;
     }
 
+    if (total_size) {
+        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
+        uint8_t *buf;
+        if (lseek(fd, 0, SEEK_SET) == -1) {
+            ret = -errno;
+        } else {
+            buf = g_malloc0(zero_size);
+            ret = qemu_write_full(fd, buf, zero_size);
+            g_free(buf);
+        }
+    }
     qemu_close(fd);
     return ret;
 }
-- 
2.13.3


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