The reason we need to copy unmodified sectors in alloc_cluster_link_l2()
is exactly to handle concurrent writes into the same cluster. This is
essentially RMW. I don't see why concurrent writes should not work with
the logic in place. There is a bug there currently of cause :) Can
somebody check this patch:
diff --git a/block-qcow2.c b/block-qcow2.c
index 7840634..801d26d 100644
--- a/block-qcow2.c
+++ b/block-qcow2.c
@@ -995,8 +995,8 @@ static int alloc_cluster_link_l2(BlockDriverState *bs,
uint64_t cluster_offset,
if(l2_table[l2_index + i] != 0)
old_cluster[j++] = l2_table[l2_index + i];
- l2_table[l2_index + i] = cpu_to_be64((cluster_offset +
- (i << s->cluster_bits)) | QCOW_OFLAG_COPIED);
+ l2_table[l2_index + i] = cpu_to_be64(((cluster_offset +
+ (i << s->cluster_bits)) | QCOW_OFLAG_COPIED));
}
if (bdrv_pwrite(s->hd, l2_offset + l2_index * sizeof(uint64_t),
@@ -1005,7 +1005,8 @@ static int alloc_cluster_link_l2(BlockDriverState *bs,
uint64_t cluster_offset,
goto err;
for (i = 0; i < j; i++)
- free_any_clusters(bs, be64_to_cpu(old_cluster[i]), 1);
+ free_any_clusters(bs, be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED,
+ 1);
ret = 0;
err: