Re: [GEDI] [PATCH 12/17] gluster: Support BDRV_ZERO_OPEN

[Niels de Vos <ndevos@redhat.com>]

On Fri, Jan 31, 2020 at 11:44:31AM -0600, Eric Blake wrote:
> Since gluster already copies file-posix for lseek usage in block
> status, it also makes sense to copy it for learning if the image
> currently reads as all zeroes.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  block/gluster.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/block/gluster.c b/block/gluster.c
> index 9d952c70981b..0417a86547c8 100644
> --- a/block/gluster.c
> +++ b/block/gluster.c
> @@ -1464,6 +1464,22 @@ exit:
>      return -ENOTSUP;
>  }
> 
> +static int qemu_gluster_known_zeroes(BlockDriverState *bs)
> +{
> +    /*
> +     * GlusterFS volume could be backed by a block device, with no way

Actually, Gluster dropped support for volumes backed by block devices
(LVM) a few releases back. Nobody could be found that used it, and it
could not be combined with other Gluster features. All contents on a
Gluster volume is now always backed by 'normal' files on a filesystem.

Creation or truncation should behave just as on a file on a local
filesystem. So maybe qemu_gluster_known_zeroes is not needed at all?

Niels


> +     * to query if regions added by creation or truncation will read
> +     * as zeroes.  However, we can use lseek(SEEK_DATA) to check if
> +     * contents currently read as zero.
> +     */
> +    off_t data, hole;
> +
> +    if (find_allocation(bs, 0, &data, &hole) == -ENXIO) {
> +        return BDRV_ZERO_OPEN;
> +    }
> +    return 0;
> +}
> +
>  /*
>   * Returns the allocation status of the specified offset.
>   *
> @@ -1561,6 +1577,7 @@ static BlockDriver bdrv_gluster = {
>      .bdrv_co_readv                = qemu_gluster_co_readv,
>      .bdrv_co_writev               = qemu_gluster_co_writev,
>      .bdrv_co_flush_to_disk        = qemu_gluster_co_flush_to_disk,
> +    .bdrv_known_zeroes            = qemu_gluster_known_zeroes,
>  #ifdef CONFIG_GLUSTERFS_DISCARD
>      .bdrv_co_pdiscard             = qemu_gluster_co_pdiscard,
>  #endif
> @@ -1591,6 +1608,7 @@ static BlockDriver bdrv_gluster_tcp = {
>      .bdrv_co_readv                = qemu_gluster_co_readv,
>      .bdrv_co_writev               = qemu_gluster_co_writev,
>      .bdrv_co_flush_to_disk        = qemu_gluster_co_flush_to_disk,
> +    .bdrv_known_zeroes            = qemu_gluster_known_zeroes,
>  #ifdef CONFIG_GLUSTERFS_DISCARD
>      .bdrv_co_pdiscard             = qemu_gluster_co_pdiscard,
>  #endif
> @@ -1621,6 +1639,7 @@ static BlockDriver bdrv_gluster_unix = {
>      .bdrv_co_readv                = qemu_gluster_co_readv,
>      .bdrv_co_writev               = qemu_gluster_co_writev,
>      .bdrv_co_flush_to_disk        = qemu_gluster_co_flush_to_disk,
> +    .bdrv_known_zeroes            = qemu_gluster_known_zeroes,
>  #ifdef CONFIG_GLUSTERFS_DISCARD
>      .bdrv_co_pdiscard             = qemu_gluster_co_pdiscard,
>  #endif
> @@ -1657,6 +1676,7 @@ static BlockDriver bdrv_gluster_rdma = {
>      .bdrv_co_readv                = qemu_gluster_co_readv,
>      .bdrv_co_writev               = qemu_gluster_co_writev,
>      .bdrv_co_flush_to_disk        = qemu_gluster_co_flush_to_disk,
> +    .bdrv_known_zeroes            = qemu_gluster_known_zeroes,
>  #ifdef CONFIG_GLUSTERFS_DISCARD
>      .bdrv_co_pdiscard             = qemu_gluster_co_pdiscard,
>  #endif
> -- 
> 2.24.1
> 
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