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docs: Document how to stream to an intermediate layer

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Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Alberto Garcia 2016-10-28 10:08:12 +03:00 committed by Kevin Wolf
parent 554b614765
commit 1029641bef
1 changed files with 25 additions and 11 deletions
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docs/live-block-ops.txt
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@ -4,15 +4,20 @@ LIVE BLOCK OPERATIONS
High level description of live block operations. Note these are not High level description of live block operations. Note these are not
supported for use with the raw format at the moment. supported for use with the raw format at the moment.
Note also that this document is incomplete and it currently only
covers the 'stream' operation. Other operations supported by QEMU such
as 'commit', 'mirror' and 'backup' are not described here yet. Please
refer to the qapi/block-core.json file for an overview of those.
Snapshot live merge Snapshot live merge
=================== ===================
Given a snapshot chain, described in this document in the following Given a snapshot chain, described in this document in the following
format: format:
[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 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 image, and so on accordingly until the leftmost image, which has no
base. base.
@ -21,11 +26,14 @@ The snapshot live merge operation transforms such a chain into a
smaller one with fewer elements, such as this transformation relative smaller one with fewer elements, such as this transformation relative
to the first example: to the first example:
[A] -> [D] [A] <- [E]
Currently only forward merge with target being the active image is Data is copied in the right direction with destination being the
supported, that is, data copy is performed in the right direction with rightmost image, but any other intermediate image can be specified
destination being the rightmost image. instead. In this example data is copied from [C] into [D], so [D] can
be backed by [B]:
[A] <- [B] <- [D] <- [E]
The operation is implemented in QEMU through image streaming facilities. The operation is implemented in QEMU through image streaming facilities.
@ -35,14 +43,20 @@ 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 active image. When finished,
it adjusts the backing file pointer. it adjusts the backing file pointer.
The 'base' parameter specifies an image which data need not be streamed from. The 'base' parameter specifies an image which data need not be
This image will be used as the backing file for the active image when the streamed from. This image will be used as the backing file for the
operation is finished. 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 (qemu) block_stream virtio0 file-A.img
In order to specify a destination image different from the active
(rightmost) one we can use its node name instead.
In the second example above, the command would be:
(qemu) block_stream node-D file-B.img
Live block copy Live block copy
=============== ===============