On 08/11/2017 03:09 AM, Fam Zheng wrote: > People get surprised when, after "qemu-img 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 confusion. > > Signed-off-by: Fam Zheng <f...@redhat.com> > > --- > > v2: Use stack allocated buffer. [Eric] > Fix return value. > (Keep qemu_write_full instead of switching to qemu_pwritev because > the former handles short writes.) > Fix typo "qemu-img". [Changlong] > --- > block/file-posix.c | 10 ++++++++++ > 1 file changed, 10 insertions(+)
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f4de022ae0..a63bbf2b90 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2703,6 +2703,16 @@ static int hdev_create(const char *filename, QemuOpts
> *opts,
> ret = -ENOSPC;
> }
>
> + if (total_size) {
> + uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
> + int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
> + if (lseek(fd, 0, SEEK_SET) == -1) {
> + ret = -errno;
> + } else {
> + ret = qemu_write_full(fd, buf, zero_size);
> + ret = ret == zero_size ? 0 : -errno;
> + }
> + }
Question: are we ever constrained by O_DIRECT where writing only 512
bytes would be too small for a block device that mandates 4k alignment?
If so, then we need MAX(minimum write size, MIN(BDRV_SECTOR_SIZE,
total_size)) - it would also mean we can't stack-allocate any more, but
that we have to do an aligned buffer allocation (where g_malloc is not
necessarily suitably aligned).
If O_DIRECT is not a problem, then this is okay:
Reviewed-by: Eric Blake <ebl...@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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