[Qemu-devel] [PATCH v2 10/11] block/backup: support bitmap sync modes for non-bitmap backups

[John Snow <jsnow@redhat.com>]

Accept bitmaps and sync policies for the other backup modes.
This allows us to do things like create a bitmap synced to a full backup
without a transaction, or start a resumable backup process.

Some combinations don't make sense, though:

- NEVER policy combined with any non-BITMAP mode doesn't do anything,
  because the bitmap isn't used for input or output.
  It's harmless, but is almost certainly never what the user wanted.

- sync=NONE is more questionable. It can't use on-success because this
  job never completes with success anyway, and the resulting artifact
  of 'always' is suspect: because we start with a full bitmap and only
  copy out segments that get written to, the final output bitmap will
  always be ... a fully set bitmap.

  Maybe there's contexts in which bitmaps make sense for sync=none,
  but not without more severe changes to the current job, and omitting
  it here doesn't prevent us from adding it later.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 block/backup.c       |  8 +-------
 blockdev.c           | 22 ++++++++++++++++++++++
 qapi/block-core.json |  6 ++++--
 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index e28fd23f6a..47628aca24 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -686,7 +686,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         return NULL;
     }
 
-    if (sync_mode == MIRROR_SYNC_MODE_BITMAP) {
+    if (sync_bitmap) {
         /* If we need to write to this bitmap, check that we can: */
         if (bitmap_mode != BITMAP_SYNC_MODE_NEVER &&
             bdrv_dirty_bitmap_check(sync_bitmap, BDRV_BITMAP_DEFAULT, errp)) {
@@ -697,12 +697,6 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
             return NULL;
         }
-    } else if (sync_bitmap) {
-        error_setg(errp,
-                   "a bitmap was given to backup_job_create, "
-                   "but it received an incompatible sync_mode (%s)",
-                   MirrorSyncMode_str(sync_mode));
-        return NULL;
     }
 
     len = bdrv_getlength(bs);
diff --git a/blockdev.c b/blockdev.c
index 3c76c85cb5..29c6c6044a 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3565,6 +3565,28 @@ static BlockJob *do_backup_common(BackupCommon *backup,
         if (bdrv_dirty_bitmap_check(bmap, BDRV_BITMAP_ALLOW_RO, errp)) {
             return NULL;
         }
+
+        /* This does not produce a useful bitmap artifact: */
+        if (backup->sync == MIRROR_SYNC_MODE_NONE) {
+            error_setg(errp, "sync mode '%s' does not produce meaningful bitmap"
+                       " outputs", MirrorSyncMode_str(backup->sync));
+            return NULL;
+        }
+
+        /* If the bitmap isn't used for input or output, this is useless: */
+        if (backup->bitmap_mode == BITMAP_SYNC_MODE_NEVER &&
+            backup->sync != MIRROR_SYNC_MODE_BITMAP) {
+            error_setg(errp, "Bitmap sync mode '%s' has no meaningful effect"
+                       " when combined with sync mode '%s'",
+                       BitmapSyncMode_str(backup->bitmap_mode),
+                       MirrorSyncMode_str(backup->sync));
+            return NULL;
+        }
+    }
+
+    if (!backup->has_bitmap && backup->has_bitmap_mode) {
+        error_setg(errp, "Cannot specify bitmap sync mode without a bitmap");
+        return NULL;
     }
 
     if (!backup->auto_finalize) {
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 5a578806c5..099e4f37b2 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1352,13 +1352,15 @@
 # @speed: the maximum speed, in bytes per second. The default is 0,
 #         for unlimited.
 #
-# @bitmap: the name of a dirty bitmap if sync is "bitmap" or "incremental".
+# @bitmap: The name of a dirty bitmap to use.
 #          Must be present if sync is "bitmap" or "incremental".
+#          Can be present if sync is "full" or "top".
 #          Must not be present otherwise.
 #          (Since 2.4 (drive-backup), 3.1 (blockdev-backup))
 #
 # @bitmap-mode: Specifies the type of data the bitmap should contain after
-#               the operation concludes. Must be present if sync is "bitmap".
+#               the operation concludes.
+#               Must be present if a bitmap was provided,
 #               Must NOT be present otherwise. (Since 4.2)
 #
 # @compress: true to compress data, if the target format supports it.
-- 
2.21.0