Am 07.06.2017 um 16:08 hat Alberto Garcia geschrieben: > If the guest tries to write data that results on the allocation of a > new cluster, instead of writing the guest data first and then the data > from the COW regions, write everything together using one single I/O > operation. > > This can improve the write performance by 25% or more, depending on > several factors such as the media type, the cluster size and the I/O > request size. > > Signed-off-by: Alberto Garcia <be...@igalia.com>
> diff --git a/block/qcow2.c b/block/qcow2.c > index b3ba5daa93..89be2083d4 100644 > --- a/block/qcow2.c > +++ b/block/qcow2.c > @@ -1575,6 +1575,44 @@ fail: > return ret; > } > > +/* 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; > + } > + > + /* The end region must be immediately after the data (middle) > + * region */ > + if (m->offset + m->cow_end.offset != offset + bytes) { > + continue; > + } > + > + /* Make sure that adding both COW regions to the QEMUIOVector > + * does not exceed IOV_MAX */ > + if (hd_qiov->niov > IOV_MAX - 2) { > + continue; > + } > + > + m->data_qiov = hd_qiov; I don't think having this side effect is good for a function that is called can_xyz(). I'd either call it merge_cow() or move the assignment to the caller. If we consider allowing to merge multiple L2Metas in the future, the actual merging could become more complicated, so maybe merge_cow() is the better option. Kevin