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

[Anthony Liguori]

On 01/27/2011 09:58 AM, Daniel P. Berrange wrote:
> On Thu, Jan 27, 2011 at 04:52:14PM +0100, Kevin Wolf wrote:
>    
> > This adds a preallocation=full mode to qcow2 image creation, which does not
> > only allocate metadata for the whole image, but also writes zeros to it,
> > creating a non-sparse image file.
> >
> > Signed-off-by: Kevin Wolf<address@hidden>
> > ---
> >   block/qcow2.c |   45 ++++++++++++++++++++++++++++++++++++++++-----
> >   1 files changed, 40 insertions(+), 5 deletions(-)
> >
> > diff --git a/block/qcow2.c b/block/qcow2.c
> > index a1773e4..90cf2ca 100644
> > --- a/block/qcow2.c
> > +++ b/block/qcow2.c
> > @@ -838,7 +838,15 @@ static int qcow2_change_backing_file(BlockDriverState *bs,
> >       return qcow2_update_ext_header(bs, backing_file, backing_fmt);
> >   }
> >
> > -static int preallocate(BlockDriverState *bs)
> > +enum prealloc_mode {
> > +    PREALLOC_OFF = 0,
> > +    PREALLOC_METADATA,
> > +    PREALLOC_FULL,
> > +};
> > +
> > +#define IO_BUF_SIZE (2 * 1024 * 1024)
> > +
> > +static int preallocate(BlockDriverState *bs, enum prealloc_mode mode)
> >   {
> >       uint64_t nb_sectors;
> >       uint64_t offset;
> > @@ -846,11 +854,14 @@ static int preallocate(BlockDriverState *bs)
> >       int ret;
> >       QCowL2Meta meta;
> >
> > +    assert(mode != PREALLOC_OFF);
> > +
> >       nb_sectors = bdrv_getlength(bs)>>  9;
> >       offset = 0;
> >       QLIST_INIT(&meta.dependent_requests);
> >       meta.cluster_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,&meta);
> > @@ -874,6 +885,28 @@ static int preallocate(BlockDriverState *bs)
> >           offset += num<<  9;
> >       }
> >
> > +    /* Then write zeros to the cluster data, if requested */
> > +    if (mode == PREALLOC_FULL) {
> > +        void *buf = qemu_mallocz(IO_BUF_SIZE);
> > +
> > +        nb_sectors = bdrv_getlength(bs)>>  BDRV_SECTOR_BITS;
> > +        offset = 0;
> > +
> > +        while (nb_sectors) {
> > +            num = MIN(nb_sectors, IO_BUF_SIZE / BDRV_SECTOR_SIZE);
> > +            ret = bdrv_write(bs, offset>>  BDRV_SECTOR_BITS, buf, num);
> >      
> Is there a way you can calculate the total size of the qcow2
> file upfront, and just use a single posix_fallocate() call to
> do the zero-filled allocation of all the data blocks. It is
> many orders of magnitude faster than truely writing blocks of
> zero'd data on modern filesystems.  I guess if you're using
> compression or encryption, we'd really have to go the slow
> path, but for regular usage it'd be better to take a fast
> path.
>    

Hrm, so is the intention here to avoid sparse files or to not assume 
zero-fill?

Regards,

Anthony Liguori

> Daniel
>
>