[Qemu-block] [PATCH v3 12/18] block/backup: add 'always' bitmap sync policy

[John Snow]

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 <address@hidden>
---
 block/backup.c       | 27 +++++++++++++++++++--------
 qapi/block-core.json |  5 ++++-
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index 3c08d057cc..b25e6179cf 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -268,18 +268,29 @@ static void backup_cleanup_sync_bitmap(BackupBlockJob 
*job, int ret)
 {
     BdrvDirtyBitmap *bm;
     BlockDriverState *bs = blk_bs(job->common.blk);
+    bool sync = (((ret == 0) || (job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS)) 
\
+                 && (job->bitmap_mode != BITMAP_SYNC_MODE_NEVER));
 
-    if (ret < 0 || job->bitmap_mode == BITMAP_SYNC_MODE_NEVER) {
+    if (sync) {
         /*
-         * Failure, or we don't want to synchronize the bitmap.
-         * Merge the successor back into the parent, delete nothing.
+         * We succeeded, or we always intended to sync the bitmap.
+         * Delete this bitmap and install the child.
          */
-        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);
+    } else {
+        /*
+         * We failed, or we never intended to sync the bitmap anyway.
+         * Merge the successor back into the parent, keeping all data.
+         */
+        bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
+    }
+
+    assert(bm);
+
+    if (ret < 0 && job->bitmap_mode == BITMAP_SYNC_MODE_ALWAYS) {
+        /* If we failed and synced, merge in the bits we didn't copy: */
+        bdrv_dirty_bitmap_merge_internal(bm, job->copy_bitmap,
+                                         NULL, true);
     }
 }
 
diff --git a/qapi/block-core.json b/qapi/block-core.json
index b1aaaaa98e..5a578806c5 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1149,10 +1149,13 @@
 # @never: The bitmap is never synchronized with the operation, and is
 #         treated solely as a read-only manifest of blocks to copy.
 #
+# @always: The bitmap is always synchronized with the operation,
+#          regardless of whether or not the operation was successful.
+#
 # Since: 4.2
 ##
 { 'enum': 'BitmapSyncMode',
-  'data': ['on-success', 'never'] }
+  'data': ['on-success', 'never', 'always'] }
 
 ##
 # @MirrorCopyMode:
-- 
2.21.0