diff mbox series

[v10,32/34] qcow2: Allow preallocation and backing files if extended_l2 is set

Message ID 87cb87b541ceccc8a6df66da0001a29e20077115.1593791819.git.berto@igalia.com (mailing list archive)
State New, archived
Headers show
Series Add subcluster allocation to qcow2 | expand

Commit Message

Alberto Garcia July 3, 2020, 3:58 p.m. UTC 
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>
---
 block/qcow2.c              | 7 ++++---
 tests/qemu-iotests/206.out | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)
diff mbox series

Patch

diff --git a/block/qcow2.c b/block/qcow2.c
index 3dfb56ccca..06e5c53b70 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3451,10 +3451,11 @@  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 363c5abe35..a100849fcb 100644
--- 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": {}}