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

[Anthony Liguori]

On 01/27/2011 09:52 AM, 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.
>    

The writing zeros bit is in order to support physical devices?  Would it 
be better to have a flag in BlockDriverState that indicated whether 
uninitialized sectors could be assumed to be zero filled and key off of 
that?

Regards,

Anthony Liguori

> 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);
> +            if (ret<  0) {
> +                qemu_free(buf);
> +                return ret;
> +            }
> +
> +            nb_sectors -= num;
> +            offset += num<<  9;
> +        }
> +
> +        qemu_free(buf);
> +    }
> +
>       /*
>        * It is expected that the image file is large enough to actually contain
>        * all of the allocated clusters (otherwise we get failing reads after
> @@ -1006,7 +1039,7 @@ static int qcow2_create2(const char *filename, int64_t 
> total_size,
>
>       /* And if we're supposed to preallocate metadata, do that now */
>       if (prealloc) {
> -        ret = preallocate(bs);
> +        ret = preallocate(bs, prealloc);
>           if (ret<  0) {
>               goto out;
>           }
> @@ -1043,9 +1076,11 @@ static int qcow2_create(const char *filename, 
> QEMUOptionParameter *options)
>               }
>           } else if (!strcmp(options->name, BLOCK_OPT_PREALLOC)) {
>               if (!options->value.s || !strcmp(options->value.s, "off")) {
> -                prealloc = 0;
> +                prealloc = PREALLOC_OFF;
>               } else if (!strcmp(options->value.s, "metadata")) {
> -                prealloc = 1;
> +                prealloc = PREALLOC_METADATA;
> +            } else if (!strcmp(options->value.s, "full")) {
> +                prealloc = PREALLOC_FULL;
>               } else {
>                   fprintf(stderr, "Invalid preallocation mode: '%s'\n",
>                       options->value.s);
> @@ -1336,7 +1371,7 @@ static QEMUOptionParameter qcow2_create_options[] = {
>       {
>           .name = BLOCK_OPT_PREALLOC,
>           .type = OPT_STRING,
> -        .help = "Preallocation mode (allowed values: off, metadata)"
> +        .help = "Preallocation mode (allowed values: off, metadata, full)"
>       },
>       { NULL }
>   };
>