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[Qemu-devel] [PATCH 3/3] docs: Document how to stream to an intermediate
[Qemu-devel] [PATCH 3/3] docs: Document how to stream to an intermediate layer
Fri, 20 Feb 2015 15:53:19 +0200
Signed-off-by: Alberto Garcia <address@hidden>
docs/live-block-ops.txt | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)
diff --git a/docs/live-block-ops.txt b/docs/live-block-ops.txt
index a257087..63530e8 100644
@@ -10,9 +10,9 @@ Snapshot live merge
Given a snapshot chain, described in this document in the following
-[A] -> [B] -> [C] -> [D]
+[A] -> [B] -> [C] -> [D] -> [E]
-Where the rightmost object ([D] in the example) described is the current
+Where the rightmost object ([E] in the example) described is the current
image which the guest OS has write access to. To the left of it is its base
image, and so on accordingly until the leftmost image, which has no
@@ -21,28 +21,38 @@ The snapshot live merge operation transforms such a chain
smaller one with fewer elements, such as this transformation relative
to the first example:
-[A] -> [D]
+[A] -> [E]
-Currently only forward merge with target being the active image is
-supported, that is, data copy is performed in the right direction with
-destination being the rightmost image.
+Data is copied in the right direction with destination being the
+rightmost image, but any other intermediate image can be specified
+instead, [D] in this example:
+[A] -> [B] -> [D] -> [E]
The operation is implemented in QEMU through image streaming facilities.
The basic idea is to execute 'block_stream virtio0' while the guest is
running. Progress can be monitored using 'info block-jobs'. When the
streaming operation completes it raises a QMP event. 'block_stream'
-copies data from the backing file(s) into the active image. When finished,
+copies data from the backing file(s) into the destination image. When finished,
it adjusts the backing file pointer.
The 'base' parameter specifies an image which data need not be streamed from.
-This image will be used as the backing file for the active image when the
+This image will be used as the backing file for the destination image when the
operation is finished.
-In the example above, the command would be:
+In the first example above, the command would be:
(qemu) block_stream virtio0 A
+The 'top' parameter specifies the destination image if it's different
+from the active (rightmost) one. Currently it can only be set using
+In the second example above this would be the QMP shell command:
+(QEMU) block-stream device=virtio0 top=D base=B
Live block copy
Re: [Qemu-devel] [PATCH 0/3] Support streaming to an intermediate layer, Eric Blake, 2015/02/20