Patchwork [v6,15/16] docs: describe live block operations

mail settings
Submitter Stefan Hajnoczi
Date Jan. 18, 2012, 2:40 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/136639/
State New
Headers show


Stefan Hajnoczi - Jan. 18, 2012, 2:40 p.m.
From: Marcelo Tosatti <>

Signed-off-by: Marcelo Tosatti <>
Signed-off-by: Stefan Hajnoczi <>
 docs/live-block-ops.txt |   58 +++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 58 insertions(+), 0 deletions(-)
 create mode 100644 docs/live-block-ops.txt


diff --git a/docs/live-block-ops.txt b/docs/live-block-ops.txt
new file mode 100644
index 0000000..a257087
--- /dev/null
+++ b/docs/live-block-ops.txt
@@ -0,0 +1,58 @@ 
+High level description of live block operations. Note these are not
+supported for use with the raw format at the moment.
+Snapshot live merge
+Given a snapshot chain, described in this document in the following
+[A] -> [B] -> [C] -> [D]
+Where the rightmost object ([D] 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
+The snapshot live merge operation transforms such a chain into a
+smaller one with fewer elements, such as this transformation relative
+to the first example:
+[A] -> [D]
+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.
+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,
+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
+operation is finished.
+In the example above, the command would be:
+(qemu) block_stream virtio0 A
+Live block copy
+To copy an in use image to another destination in the filesystem, one
+should create a live snapshot in the desired destination, then stream
+into that image. Example:
+(qemu) snapshot_blkdev ide0-hd0 /new-path/disk.img qcow2
+(qemu) block_stream ide0-hd0