live migration: Serialize vmstate saving in stage 2

The effect of this patch with current block migration is that its stage
2, ie. the first full walk-through of the block devices will be
performed completely before RAM migration starts. This ensures that
continuously changing RAM pages are not re-synchronized all the time
while block migration is not completed.

Future versions of block migration which will respect the specified
downtime will generate a different pattern: After RAM migration has
started as well, block migration may also continue to inject dirty
blocks into the RAM stream once it detects that the number of pending
blocks would extend the downtime unacceptably.

Note that all this relies on the current registration order: block
before RAM migration.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

diff --git a/savevm.c b/savevm.c
index db44ed607b4d9ea44e6ca92b6f409f1f398276f5..aefe052cd3b691072887b49468c78512d1c967ff 100644 (file)
--- a/savevm.c
+++ b/savevm.c
@@ -1321,7 +1321,14 @@ int qemu_savevm_state_iterate(Monitor *mon, QEMUFile *f)
         qemu_put_byte(f, QEMU_VM_SECTION_PART);
         qemu_put_be32(f, se->section_id);
 
-        ret &= !!se->save_live_state(mon, f, QEMU_VM_SECTION_PART, se->opaque);
+        ret = se->save_live_state(mon, f, QEMU_VM_SECTION_PART, se->opaque);
+        if (!ret) {
+            /* Do not proceed to the next vmstate before this one reported
+               completion of the current stage. This serializes the migration
+               and reduces the probability that a faster changing state is
+               synchronized over and over again. */
+            break;
+        }
     }
 
     if (ret)