qcow2: Batch flushes for COW

qcow2 calls bdrv_flush() after performing COW in order to ensure that the
L2 table change is never written before the copy is safe on disk. Now that the
L2 table is cached, we can wait with flushing until we write out the next L2
table.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 19 insertions, 4 deletions

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 5f1740b9a..8f2955b37 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -38,6 +38,7 @@ struct Qcow2Cache {
     int                     size;
     Qcow2CachedTable*       entries;
     struct Qcow2Cache*      depends;
+    bool                    depends_on_flush;
     bool                    writethrough;
 };
 
@@ -85,13 +86,15 @@ static int qcow2_cache_flush_dependency(BlockDriverState *bs, Qcow2Cache *c)
     }
 
     c->depends = NULL;
+    c->depends_on_flush = false;
+
     return 0;
 }
 
 static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
 {
     BDRVQcowState *s = bs->opaque;
-    int ret;
+    int ret = 0;
 
     if (!c->entries[i].dirty || !c->entries[i].offset) {
         return 0;
@@ -99,11 +102,17 @@ static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
 
     if (c->depends) {
         ret = qcow2_cache_flush_dependency(bs, c);
-        if (ret < 0) {
-            return ret;
+    } else if (c->depends_on_flush) {
+        ret = bdrv_flush(bs->file);
+        if (ret >= 0) {
+            c->depends_on_flush = false;
         }
     }
 
+    if (ret < 0) {
+        return ret;
+    }
+
     if (c == s->refcount_block_cache) {
         BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_UPDATE_PART);
     } else if (c == s->l2_table_cache) {
@@ -167,6 +176,11 @@ int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     return 0;
 }
 
+void qcow2_cache_depends_on_flush(Qcow2Cache *c)
+{
+    c->depends_on_flush = true;
+}
+
 static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
 {
     int i;
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 76e7e07bd..1c2003a8a 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -637,7 +637,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
      * handled.
      */
     if (cow) {
-        bdrv_flush(bs->file);
+        qcow2_cache_depends_on_flush(s->l2_table_cache);
     }
 
     qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache);
diff --git a/block/qcow2.h b/block/qcow2.h
index 11cbce337..6d801206e 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -229,6 +229,7 @@ void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
 int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
 int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency);
+void qcow2_cache_depends_on_flush(Qcow2Cache *c);
 
 int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
     void **table);