[PATCH] qemu: backup: Install bitmap for incremental backup to appropriate node only

[Peter Krempa]

Libvirt's backup code has two modes:

1) push - where qemu actively writes the difference since the checkpoint
          into the output file

2) pull - where we instruct qemu to expose a frozen disk state along
          with a bitmap of blocks which changed since the checkpoint

For push mode qemu needs the temporary bitmap we use where we calculate
the actual changes to be present on the block node backing the disk.

For pull mode where we expose the bitmap via NBD qemu actually wants the
bitmap to be present for the exported block node which is the scratch
file.

Until now we've calculated the bitmap twice and installed it both to the
scratch file and to the disk node, but we don't need to since we know
when it's needed.

Pass in the 'pull' flag and decide where to install the bitmap according
to it and also when to register the bitmap name with the blockjob.

Signed-off-by: Peter Krempa <pkrempa at redhat.com>
---
 src/qemu/qemu_backup.c | 42 ++++++++++++++++++++++++++----------------
 1 file changed, 26 insertions(+), 16 deletions(-)

diff --git a/src/qemu/qemu_backup.c b/src/qemu/qemu_backup.c
index 99dee46c4f..09f7921ea7 100644
--- a/src/qemu/qemu_backup.c
+++ b/src/qemu/qemu_backup.c
@@ -109,6 +109,7 @@ struct qemuBackupDiskData {
     virStorageSourcePtr terminator;
     virStorageSourcePtr backingStore;
     char *incrementalBitmap;
+    const char *domdiskIncrementalBitmap; /* name of temporary bitmap installed on disk source */
     qemuBlockStorageSourceChainDataPtr crdata;
     bool labelled;
     bool initialized;
@@ -201,6 +202,7 @@ qemuBackupDiskPrepareOneBitmapsChain(virStorageSourcePtr backingChain,
 static int
 qemuBackupDiskPrepareOneBitmaps(struct qemuBackupDiskData *dd,
                                 virJSONValuePtr actions,
+                                bool pull,
                                 GHashTable *blockNamedNodeData)
 {
     if (!qemuBlockBitmapChainIsValid(dd->domdisk->src,
@@ -212,21 +214,29 @@ qemuBackupDiskPrepareOneBitmaps(struct qemuBackupDiskData *dd,
         return -1;
     }

-    if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
-                                             dd->domdisk->src,
-                                             dd->incrementalBitmap,
-                                             dd->backupdisk->incremental,
-                                             actions,
-                                             blockNamedNodeData) < 0)
-        return -1;
+    /* For pull-mode backup, we need the bitmap to be present in the scratch
+     * file as that will be exported. For push-mode backup the bitmap is
+     * actually required on top of the image backing the disk */

-    if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
-                                             dd->store,
-                                             dd->incrementalBitmap,
-                                             dd->backupdisk->incremental,
-                                             actions,
-                                             blockNamedNodeData) < 0)
-        return -1;
+    if (pull) {
+        if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
+                                                 dd->store,
+                                                 dd->incrementalBitmap,
+                                                 dd->backupdisk->incremental,
+                                                 actions,
+                                                 blockNamedNodeData) < 0)
+            return -1;
+    } else {
+        if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
+                                                 dd->domdisk->src,
+                                                 dd->incrementalBitmap,
+                                                 dd->backupdisk->incremental,
+                                                 actions,
+                                                 blockNamedNodeData) < 0)
+            return -1;
+
+        dd->domdiskIncrementalBitmap = dd->backupdisk->incremental;
+    }

     return 0;
 }
@@ -293,12 +303,12 @@ qemuBackupDiskPrepareDataOne(virDomainObjPtr vm,
         else
             dd->incrementalBitmap = g_strdup_printf("backup-%s", dd->domdisk->dst);

-        if (qemuBackupDiskPrepareOneBitmaps(dd, actions, blockNamedNodeData) < 0)
+        if (qemuBackupDiskPrepareOneBitmaps(dd, actions, pull, blockNamedNodeData) < 0)
             return -1;
     }

     if (!(dd->blockjob = qemuBlockJobDiskNewBackup(vm, dd->domdisk, dd->store,
-                                                   dd->incrementalBitmap)))
+                                                   dd->domdiskIncrementalBitmap)))
         return -1;

     /* use original disk as backing to prevent opening the backing chain */
-- 
2.28.0