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

["Daniel P. Berrange" <berrange@redhat.com>]

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 <kwolf@redhat.com>
> ---
>  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.

Daniel