On 27.04.2017 03:46, Eric Blake wrote: > Similar to discard_single_l2(), we should try to avoid dirtying > the L2 cache when the cluster we are changing already has the > right characteristics. > > Note that by the time we get to zero_single_l2(), BDRV_REQ_MAY_UNMAP > is a requirement to unallocate a cluster (this is because the block > layer clears that flag if discard.* flags during open requested that > we never punch holes - see the conversation around commit 170f4b2e, > https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg07306.html). > Therefore, this patch can only reuse a zero cluster as-is if either > unmapping is not requested, or if the zero cluster was not associated > with an allocation. > > Technically, there are some cases where an unallocated cluster > already reads as all zeroes (namely, when there is no backing file > [easy: check bs->backing], or when the backing file also reads as > zeroes [harder: we can't check bdrv_get_block_status since we are > already holding the lock]), where the guest would not immediately see > a difference if we left that cluster unallocated. But if the user > did not request unmapping, leaving an unallocated cluster is wrong; > and even if the user DID request unmapping, keeping a cluster > unallocated risks a subtle semantic change of guest-visible contents > if a backing file is later added, and it is not worth auditing > whether all internal uses such as mirror properly avoid an unmap > request. Thus, this patch is intentionally limited to just clusters > that are already marked as zero. > > Signed-off-by: Eric Blake <ebl...@redhat.com> > > --- > v10: new patch, replacing earlier attempt to use unallocated clusters, > and ditching any optimization of v2 files > --- > block/qcow2-cluster.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index db3d937..d542894 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -1614,6 +1614,7 @@ static int zero_single_l2(BlockDriverState *bs, > uint64_t offset, > int l2_index; > int ret; > int i; > + bool unmap = !!(flags & BDRV_REQ_MAY_UNMAP); > > ret = get_cluster_table(bs, offset, &l2_table, &l2_index); > if (ret < 0) { > @@ -1629,9 +1630,17 @@ static int zero_single_l2(BlockDriverState *bs, > uint64_t offset, > > old_offset = be64_to_cpu(l2_table[l2_index + i]); > > - /* Update L2 entries */ > + /* > + * Minimize L2 changes if the cluster already reads back as > + * zeroes with correct allocation. > + */ > + if (qcow2_get_cluster_type(old_offset) == QCOW2_CLUSTER_ZERO && > + !(unmap && old_offset & L2E_OFFSET_MASK)) { > + continue; > + } > + > qcow2_cache_entry_mark_dirty(bs, s->l2_table_cache, l2_table); > - if (old_offset & QCOW_OFLAG_COMPRESSED || flags & > BDRV_REQ_MAY_UNMAP) { > + if (old_offset & QCOW_OFLAG_COMPRESSED || unmap) {
Works, but I'd like it better to store the cluster type somewhere and
then use it here instead of checking the COMPRESSED flag.
But it works, so it's up to you:
Reviewed-by: Max Reitz <mre...@redhat.com>
> l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
> qcow2_free_any_clusters(bs, old_offset, 1,
> QCOW2_DISCARD_REQUEST);
> } else {
>
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