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[Qemu-devel] [PATCH V4 3/4] Add backing drive while performing backup.

From: Ian Main
Subject: [Qemu-devel] [PATCH V4 3/4] Add backing drive while performing backup.
Date: Wed, 17 Jul 2013 13:04:21 -0700

This patch adds the original source drive as a backing drive to our target
image so that the target image will appear complete during backup.  This
is especially useful for SYNC_MODE_NONE as it allows export via NBD to
have a complete point-in-time snapshot available for export.

Signed-off-by: Ian Main <address@hidden>
 block/backup.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/block/backup.c b/block/backup.c
index 68abd23..e086e76 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -323,6 +323,10 @@ static void coroutine_fn backup_run(void *opaque)
+    /* Set the target backing drive back to NULL before calling delete or
+     * it will also delete the underlying drive. */
+    target->backing_hd = NULL;
@@ -362,6 +366,15 @@ void backup_start(BlockDriverState *bs, BlockDriverState 
+    /* Manually set the backing hd to be the backup source drive so
+     * that all reads done while we are backing up will be passed
+     * on to the original source drive.  This allows reading from the
+     * image while the backup is in progress, or in the case of
+     * SYNC_MODE_NONE allows a complete image to be present for export.
+     * Note that we do this for all modes including SYNC_MODE_TOP as
+     * even then it allows on-the-fly reading. */
+    target->backing_hd = bs;
     job->on_source_error = on_source_error;
     job->on_target_error = on_target_error;
     job->target = target;

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