Re: [Qemu-devel] [RFC PATCH 4/4] qcow2: Add full image preallocation option

[Hu Tao]

On Wed, Nov 13, 2013 at 07:03:03AM +0100, Peter Lieven wrote:
> What is your use case for this seris? QCOW2 creation or converting
> anything to QCOW2? For the later case you could use "qemu-img convert -S 0
> ..."
> starting in 1.8.

The former. Seems "qemu-img convert -S 0" do the same thing but at
converting time, maybe we can share some code. But let me dig the
code first.

> 
> Peter
> 
> Hu Tao wrote:
> > This adds a preallocation=full mode to qcow2 image creation, which
> > creates a non-sparse image file.
> >
> > Signed-off-by: Hu Tao <address@hidden>
> > ---
> >  block/qcow2.c | 28 ++++++++++++++++++++++------
> >  1 file changed, 22 insertions(+), 6 deletions(-)
> >
> > diff --git a/block/qcow2.c b/block/qcow2.c
> > index 359030f..d3ca6cf 100644
> > --- a/block/qcow2.c
> > +++ b/block/qcow2.c
> > @@ -1385,7 +1385,13 @@ static int
> > qcow2_change_backing_file(BlockDriverState *bs,
> >      return qcow2_update_header(bs);
> >  }
> >
> > -static int preallocate(BlockDriverState *bs)
> > +enum prealloc_mode {
> > +    PREALLOC_OFF = 0,
> > +    PREALLOC_METADATA,
> > +    PREALLOC_FULL,
> > +};
> > +
> > +static int preallocate(BlockDriverState *bs, enum prealloc_mode mode)
> >  {
> >      uint64_t nb_sectors;
> >      uint64_t offset;
> > @@ -1394,9 +1400,12 @@ static int preallocate(BlockDriverState *bs)
> >      int ret;
> >      QCowL2Meta *meta;
> >
> > +    assert(mode != PREALLOC_OFF);
> > +
> >      nb_sectors = bdrv_getlength(bs) >> 9;
> >      offset = 0;
> >
> > +    /* First allocate metadata in _really_ big chunks */
> >      while (nb_sectors) {
> >          num = MIN(nb_sectors, INT_MAX >> 9);
> >          ret = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num,
> > @@ -1424,6 +1433,11 @@ static int preallocate(BlockDriverState *bs)
> >          offset += num << 9;
> >      }
> >
> > +    /* Then write zeros to the cluster data, if requested */
> > +    if (mode == PREALLOC_FULL) {
> > +        bdrv_zero_init(bs->file, offset, bdrv_getlength(bs));
> > +    }
> > +
> >      /*
> >       * It is expected that the image file is large enough to actually
> > contain
> >       * all of the allocated clusters (otherwise we get failing reads
> > after
> > @@ -1572,11 +1586,11 @@ static int qcow2_create2(const char *filename,
> > int64_t total_size,
> >          }
> >      }
> >
> > -    /* And if we're supposed to preallocate metadata, do that now */
> > +    /* And if we're supposed to preallocate data, do that now */
> >      if (prealloc) {
> >          BDRVQcowState *
>