Re: [Qemu-block] [PATCH 7/7] qcow2: Merge the writing of the COW regions with the guest data

[Alberto Garcia]

On Tue 23 May 2017 01:23:02 PM CEST, Alberto Garcia wrote:

You can still review this now if you want, but here's a couple of minor
things I'll correct in the next revision:

> +    if (m->data_qiov) {
> +        qemu_iovec_reset(&qiov);
> +        qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
> +        qemu_iovec_concat(&qiov, m->data_qiov, 0, data_bytes);
> +        qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);

I'll check start->nb_bytes and end->nb_bytes for zero before calling
qemu_iovec_add() (no practical difference, but I find it a bit cleaner).

> +/* Check if it's possible to merge a write request with the writing of
> + * the data from the COW regions */
> +static bool can_merge_cow(uint64_t offset, unsigned bytes,
> +                          QEMUIOVector *hd_qiov, QCowL2Meta *l2meta)
> +{
> +    QCowL2Meta *m;
> +
> +    for (m = l2meta; m != NULL; m = m->next) {
> +        /* If both COW regions are empty then there's nothing to merge */
> +        if (m->cow_start.nb_bytes == 0 && m->cow_end.nb_bytes == 0) {
> +            continue;
> +        }
> +
> +        /* The data (middle) region must be immediately after the
> +         * start region */
> +        if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
> +            continue;
> +        }

I realized that I'm using guest offsets in this function. Since this
checks if we can _write_ everything together I think I should be using
host offsets instead.

I also don't think this makes any difference in practice in this case,
but I'll update it.

Berto