On 08/10/2017 03:01 AM, Fam Zheng wrote:
> 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;

Since BDRV_SECTOR_SIZE is small enough to stack-allocate, you could skip
the malloc by doing:

uint8_t buf[BDRV_SECTOR_SIZE] = "";

> +        if (lseek(fd, 0, SEEK_SET) == -1) {
> +            ret = -errno;
> +        } else {
> +            buf = g_malloc0(zero_size);
> +            ret = qemu_write_full(fd, buf, zero_size);

Instead of doing lseek + qemu_write_full, can we just use
qemu_pwritev(fd, &iov, 1, 0) with an iov set up to point to the
appropriate amount of buf?

At any rate, my ideas are micro-optimizations, so I can also live with
how you wrote it.

Reviewed-by: Eric Blake <ebl...@redhat.com>

Are you arguing that this is a bug-fix worthy of inclusion in 2.10,
because it helps avoid user confusion? Or are you delaying it to 2.11,
because we've had the existing behavior for longer than one release, so
one release more won't hurt?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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