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>

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) {
+    } else if (c->depends_on_flush) {
-            return ret;
+        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;

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);

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);