[Qemu-devel] [PATCH 07/12] block/backup: add 'always' bitmap sync policy

[John Snow <jsnow@redhat.com>]

This adds an "always" policy for bitmap synchronization. Regardless of if
the job succeeds or fails, the bitmap is *always* synchronized. This means
that for backups that fail part-way through, the bitmap retains a record of
which sectors need to be copied out to accomplish a new backup using the
old, partial result.

In effect, this allows us to "resume" a failed backup; however the new backup
will be from the new point in time, so it isn't a "resume" as much as it is
an "incremental retry." This can be useful in the case of extremely large
backups that fail considerably through the operation and we'd like to not waste
the work that was already performed.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 qapi/block-core.json |  5 ++++-
 block/backup.c       | 10 ++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 0332dcaabc..58d267f1f5 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1143,6 +1143,9 @@
 # An enumeration of possible behaviors for the synchronization of a bitmap
 # when used for data copy operations.
 #
+# @always: The bitmap is always synchronized with remaining blocks to copy,
+#          whether or not the operation has completed successfully or not.
+#
 # @conditional: The bitmap is only synchronized when the operation is successul.
 #               This is useful for Incremental semantics.
 #
@@ -1153,7 +1156,7 @@
 # Since: 4.1
 ##
 { 'enum': 'BitmapSyncMode',
-  'data': ['conditional', 'never'] }
+  'data': ['always', 'conditional', 'never'] }
 
 ##
 # @MirrorCopyMode:
diff --git a/block/backup.c b/block/backup.c
index 627f724b68..beb2078696 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -266,15 +266,17 @@ static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
     BlockDriverState *bs = blk_bs(job->common.blk);
 
     if (ret < 0 || job->bitmap_mode == BITMAP_SYNC_MODE_NEVER) {
-        /* Failure, or we don't want to synchronize the bitmap.
-         * Merge the successor back into the parent, delete nothing. */
+        /* Failure, or we don't want to synchronize the bitmap. */
+        if (job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS) {
+            bdrv_dirty_bitmap_claim(job->sync_bitmap, &job->copy_bitmap);
+        }
+        /* Merge the successor back into the parent. */
         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
-        assert(bm);
     } else {
         /* Everything is fine, delete this bitmap and install the backup. */
         bm = bdrv_dirty_bitmap_abdicate(bs, job->sync_bitmap, NULL);
-        assert(bm);
     }
+    assert(bm);
 }
 
 static void backup_commit(Job *job)
-- 
2.21.0