73 lines
2.3 KiB
Plaintext
73 lines
2.3 KiB
Plaintext
LIVE BLOCK OPERATIONS
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=====================
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High level description of live block operations. Note these are not
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supported for use with the raw format at the moment.
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Note also that this document is incomplete and it currently only
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covers the 'stream' operation. Other operations supported by QEMU such
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as 'commit', 'mirror' and 'backup' are not described here yet. Please
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refer to the qapi/block-core.json file for an overview of those.
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Snapshot live merge
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===================
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Given a snapshot chain, described in this document in the following
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format:
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[A] <- [B] <- [C] <- [D] <- [E]
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Where the rightmost object ([E] in the example) described is the current
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image which the guest OS has write access to. To the left of it is its base
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image, and so on accordingly until the leftmost image, which has no
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base.
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The snapshot live merge operation transforms such a chain into a
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smaller one with fewer elements, such as this transformation relative
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to the first example:
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[A] <- [E]
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Data is copied in the right direction with destination being the
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rightmost image, but any other intermediate image can be specified
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instead. In this example data is copied from [C] into [D], so [D] can
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be backed by [B]:
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[A] <- [B] <- [D] <- [E]
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The operation is implemented in QEMU through image streaming facilities.
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The basic idea is to execute 'block_stream virtio0' while the guest is
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running. Progress can be monitored using 'info block-jobs'. When the
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streaming operation completes it raises a QMP event. 'block_stream'
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copies data from the backing file(s) into the active image. When finished,
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it adjusts the backing file pointer.
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The 'base' parameter specifies an image which data need not be
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streamed from. This image will be used as the backing file for the
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destination image when the operation is finished.
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In the first example above, the command would be:
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(qemu) block_stream virtio0 file-A.img
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In order to specify a destination image different from the active
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(rightmost) one we can use its node name instead.
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In the second example above, the command would be:
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(qemu) block_stream node-D file-B.img
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Live block copy
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===============
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To copy an in use image to another destination in the filesystem, one
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should create a live snapshot in the desired destination, then stream
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into that image. Example:
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(qemu) snapshot_blkdev ide0-hd0 /new-path/disk.img qcow2
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(qemu) block_stream ide0-hd0
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