qcow2: skip writing zero buffers to empty COW areas

If COW areas of the newly allocated clusters are zeroes on the backing
image, efficient bdrv_write_zeroes(flags=BDRV_REQ_NO_FALLBACK) can be
used on the whole cluster instead of writing explicit zero buffers later
in perform_cow().

iotest 060:
write to the discarded cluster does not trigger COW anymore.
Use a backing image instead.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Message-id: 20190516142749.81019-2-anton.nefedov@virtuozzo.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 85 insertions, 0 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index dea765b..f2cb131 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2103,6 +2103,11 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
             continue;
         }
 
+        /* If COW regions are handled already, skip this too */
+        if (m->skip_cow) {
+            continue;
+        }
+
         /* The data (middle) region must be immediately after the
          * start region */
         if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
@@ -2128,6 +2133,80 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
     return false;
 }
 
+static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t bytes)
+{
+    int64_t nr;
+    return !bytes ||
+        (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr == bytes);
+}
+
+static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
+{
+    /*
+     * This check is designed for optimization shortcut so it must be
+     * efficient.
+     * Instead of is_zero(), use is_unallocated() as it is faster (but not
+     * as accurate and can result in false negatives).
+     */
+    return is_unallocated(bs, m->offset + m->cow_start.offset,
+                          m->cow_start.nb_bytes) &&
+           is_unallocated(bs, m->offset + m->cow_end.offset,
+                          m->cow_end.nb_bytes);
+}
+
+static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
+{
+    BDRVQcow2State *s = bs->opaque;
+    QCowL2Meta *m;
+
+    if (!(s->data_file->bs->supported_zero_flags & BDRV_REQ_NO_FALLBACK)) {
+        return 0;
+    }
+
+    if (bs->encrypted) {
+        return 0;
+    }
+
+    for (m = l2meta; m != NULL; m = m->next) {
+        int ret;
+
+        if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
+            continue;
+        }
+
+        if (!is_zero_cow(bs, m)) {
+            continue;
+        }
+
+        /*
+         * instead of writing zero COW buffers,
+         * efficiently zero out the whole clusters
+         */
+
+        ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
+                                            m->nb_clusters * s->cluster_size,
+                                            true);
+        if (ret < 0) {
+            return ret;
+        }
+
+        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
+        ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset,
+                                    m->nb_clusters * s->cluster_size,
+                                    BDRV_REQ_NO_FALLBACK);
+        if (ret < 0) {
+            if (ret != -ENOTSUP && ret != -EAGAIN) {
+                return ret;
+            }
+            continue;
+        }
+
+        trace_qcow2_skip_cow(qemu_coroutine_self(), m->offset, m->nb_clusters);
+        m->skip_cow = true;
+    }
+    return 0;
+}
+
 static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
                                          uint64_t bytes, QEMUIOVector *qiov,
                                          int flags)
@@ -2207,6 +2286,12 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
             qemu_iovec_add(&hd_qiov, cluster_data, cur_bytes);
         }
 
+        /* Try to efficiently initialize the physical space with zeroes */
+        ret = handle_alloc_space(bs, l2meta);
+        if (ret < 0) {
+            goto out_unlocked;
+        }
+
         /* If we need to do COW, check if it's possible to merge the
          * writing of the guest data together with that of the COW regions.
          * If it's not possible (or not necessary) then write the