[Qemu-devel] [PATCH 2/2] block: resize backing image during active layer commit, if needed

[Jeff Cody]

If the top image to commit is the active layer, and also larger than
the base image, then an I/O error will likely be returned during
block-commit.

For instance, if we have a base image with a virtual size 10G, and a
active layer image of size 20G, then committing the snapshot via
'block-commit' will likely fail.

This will automatically attempt to resize the base image, if the
active layer image to be committed is larger.

Signed-off-by: Jeff Cody <jc...@redhat.com>
---
 block/mirror.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 2932bab..c4e42fa 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -630,9 +630,22 @@ void commit_active_start(BlockDriverState *bs, 
BlockDriverState *base,
                          BlockDriverCompletionFunc *cb,
                          void *opaque, Error **errp)
 {
+    int64_t length;
     if (bdrv_reopen(base, bs->open_flags, errp)) {
         return;
     }
+
+    length = bdrv_getlength(bs);
+
+    if (length > bdrv_getlength(base)) {
+        if (bdrv_truncate(base, length) < 0) {
+            error_setg(errp, "Top image %s is larger than base image %s, and "
+                             "resize of base image failed.",
+                             bs->filename, base->filename);
+            return;
+        }
+    }
+
     bdrv_ref(base);
     mirror_start_job(bs, base, speed, 0, 0,
                      on_error, on_error, cb, opaque, errp,
-- 
1.8.3.1