On Fri, 08/11 09:42, Eric Blake wrote:
> 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:
A few lines above:
fd = qemu_open(filename, O_WRONLY | O_BINARY);
so there is no O_DIRECT issue.
Fam
