Re: [PATCH v6 04/10] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Eric Blake <eblake@redhat.com>]

On 4/23/20 10:01 AM, 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>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>   block/qcow2-cluster.c |  2 +-
>   block/qcow2.c         | 33 +++++++++++++++++++++++++++++++++
>   2 files changed, 34 insertions(+), 1 deletion(-)
> 

> +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
> +
> +        /*
> +         * Use zero clusters as much as we can. qcow2_cluster_zeroize()
> +         * requires a cluster-aligned start. The end may be unaligned if it is
> +         * at the end of the image (which it is here).
> +         */
> +        ret = qcow2_cluster_zeroize(bs, zero_start, offset - zero_start, 0);
> +        if (ret < 0) {
> +            error_setg_errno(errp, -ret, "Failed to zero out new clusters");
> +            goto fail;
> +        }
> +
> +        /* Write explicit zeros for the unaligned head */
> +        if (zero_start > old_length) {
> +            uint8_t *buf = qemu_blockalign0(bs, s->cluster_size);
> +            QEMUIOVector qiov;
> +            qemu_iovec_init_buf(&qiov, buf, zero_start - old_length);
> +
> +            qemu_co_mutex_unlock(&s->lock);
> +            ret = qcow2_co_pwritev_part(bs, old_length, qiov.size, &qiov, 0, 0);

This works, but would it be any more efficient to use 
qcow2_co_pwrite_zeroes?  If the head of the cluster is already zero, 
then qcow2_co_pwrite_zeroes can turn into qcow2_cluster_zeroize for this 
cluster, while qcow2_co_pwritev_part cannot.

Because what you have works, and because we can use 
qcow2_co_pwrite_zeroes as an optimization in a later patch,

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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