Re: [Qemu-devel] [PATCH for-2.11 v2] file-posix: Clear out first sector in hdev_create

[Fam Zheng <famz@redhat.com>]

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 <famz@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