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[PATCH] qcow2: flush qcow2 l2 meta for new allocated clusters

From: Ying Fang
Subject: [PATCH] qcow2: flush qcow2 l2 meta for new allocated clusters
Date: Wed, 5 Aug 2020 10:38:26 +0800

From: fangying <fangying1@huawei.com>

When qemu or qemu-nbd process uses a qcow2 image and configured with
'cache = none', it will write to the qcow2 image with a cache to cache
L2 tables, however the process will not use L2 tables without explicitly
calling the flush command or closing the mirror flash into the disk.
Which may cause the disk data inconsistent with the written data for
a long time. If an abnormal process exit occurs here, the issued written
data will be lost.

Therefore, in order to keep data consistency we need to flush the changes
to the L2 entry to the disk in time for the newly allocated cluster.

Signed-off-by: Ying Fang <fangying1@huawei.com>

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 7444b9c..ab6e812 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -266,6 +266,22 @@ int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
     return result;
+int qcow2_cache_l2_write_entry(BlockDriverState *bs, Qcow2Cache *c,
+                               void *table, int index, int num)
+    int ret;
+    int i = qcow2_cache_get_table_idx(c, table);
+    int start_sector = index / L2_ENTRIES_PER_SECTOR;
+    int end_sector = (index + num - 1) / L2_ENTRIES_PER_SECTOR;
+    int nr_sectors = end_sector - start_sector + 1;
+    ret = bdrv_pwrite(bs->file,
+                      c->entries[i].offset + start_sector * BDRV_SECTOR_SIZE,
+                      table + start_sector * BDRV_SECTOR_SIZE,
+                      nr_sectors * BDRV_SECTOR_SIZE);
+    return ret;
 int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index a677ba9..ae49a83 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -998,6 +998,9 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, 
QCowL2Meta *m)
+    ret = qcow2_cache_l2_write_entry(bs, s->l2_table_cache, l2_slice,
+                                     l2_index, m->nb_clusters);
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
diff --git a/block/qcow2.h b/block/qcow2.h
index 7ce2c23..168ab59 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -748,6 +748,8 @@ int qcow2_cache_destroy(Qcow2Cache *c);
 void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
 int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
 int qcow2_cache_write(BlockDriverState *bs, Qcow2Cache *c);
+int qcow2_cache_l2_write_entry(BlockDriverState *bs, Qcow2Cache *c,
+                               void *table, int index, int num);
 int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency);
 void qcow2_cache_depends_on_flush(Qcow2Cache *c);

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