qcow2: Allow preallocation and backing files if extended_l2 is set

Traditional qcow2 images don't allow preallocation if a backing file
is set. This is because once a cluster is allocated there is no way to
tell that its data should be read from the backing file.

Extended L2 entries have individual allocation bits for each
subcluster, and therefore it is perfectly possible to have an
allocated cluster with all its subclusters unallocated.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <6d5b0f38e7dc5f2f31d8cab1cb92044e9909aece.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

diff --git a/block/qcow2.c b/block/qcow2.c
index 00cda5696b82fba8c6061464627bf9550ba2b1e6..da56b1a4df4fdf51cf6608d04c975b2c22457ac4 100644 (file)
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3449,10 +3449,11 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
         qcow2_opts->preallocation = PREALLOC_MODE_OFF;
     }
     if (qcow2_opts->has_backing_file &&
-        qcow2_opts->preallocation != PREALLOC_MODE_OFF)
+        qcow2_opts->preallocation != PREALLOC_MODE_OFF &&
+        !qcow2_opts->extended_l2)
     {
-        error_setg(errp, "Backing file and preallocation cannot be used at "
-                   "the same time");
+        error_setg(errp, "Backing file and preallocation can only be used at "
+                   "the same time if extended_l2 is on");
         ret = -EINVAL;
         goto out;
     }

diff --git a/tests/qemu-iotests/206.out b/tests/qemu-iotests/206.out
index 363c5abe3551a7df133dc919c0d9a1a1e6288cbc..a100849fcb487daa91ea39f5332ee9fc67a15f23 100644 (file)
--- a/tests/qemu-iotests/206.out
+++ b/tests/qemu-iotests/206.out
@@ -203,7 +203,7 @@ Job failed: Different refcount widths than 16 bits require compatibility level 1
 === Invalid backing file options ===
 {"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"backing-file": "/dev/null", "driver": "qcow2", "file": "node0", "preallocation": "full", "size": 67108864}}}
 {"return": {}}
-Job failed: Backing file and preallocation cannot be used at the same time
+Job failed: Backing file and preallocation can only be used at the same time if extended_l2 is on
 {"execute": "job-dismiss", "arguments": {"id": "job0"}}
 {"return": {}}