Re: [PATCH v7 04/10] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Max Reitz <mreitz@redhat.com>]

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On 24.04.20 14:54, Kevin Wolf wrote:
> If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> undo any previous preallocation, but just adds the zero flag to all
> relevant L2 entries. If an external data file is in use, a write_zeroes
> request to the data file is made instead.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2-cluster.c |  2 +-
>  block/qcow2.c         | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+), 1 deletion(-)

[...]

> diff --git a/block/qcow2.c b/block/qcow2.c
> index 9cfbdfc939..98065d7808 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c

[...]

> @@ -4214,6 +4215,39 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,

[...]

> +        /* Write explicit zeros for the unaligned head */
> +        if (zero_start > old_length) {
> +            uint64_t len = zero_start - old_length;
> +            uint8_t *buf = qemu_blockalign0(bs, len);

I wonder whether I should raise the question of why this should be
block-aligned when we make no effort to align the offset its written to
(and we know it isn’t aligned to qcow2 clusters at least).

I probably should not.

Reviewed-by: Max Reitz <mreitz@redhat.com>

> +            QEMUIOVector qiov;
> +            qemu_iovec_init_buf(&qiov, buf, len);
> +
> +            qemu_co_mutex_unlock(&s->lock);
> +            ret = qcow2_co_pwritev_part(bs, old_length, len, &qiov, 0, 0);
> +            qemu_co_mutex_lock(&s->lock);
> +
> +            qemu_vfree(buf);
> +            if (ret < 0) {
> +                error_setg_errno(errp, -ret, "Failed to zero out the new area");
> +                goto fail;
> +            }
> +        }


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