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docs: Migration feature document
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Acked-by: Ian Campbell <ian.campbell@citrix.com>
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% Migration
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% Revision 1
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\clearpage
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# Basics
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--------------- -------------
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Status: **Supported**
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Architecture: x86
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Component: Toolstack
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--------------- -------------
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# Overview
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Migration is a mechanism to move a virtual machine while the VM is
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running. Live migration moves a running virtual machine between two
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physical servers, but the same mechanism can be used for non-live
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migration (pause and copy) and suspend/resume from disk.
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# User details
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No hardware requirements, although hypervisor logdirty support is
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required for live migration.
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From the command line, `xl migrate/save/restore` are the top level
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interactions. e.g.
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xl create my-vm.cfg
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xl migrate my-vm localhost
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or
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xl create my-vm.cfg
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xl save my-vm /path/to/save/file
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xl restore /path/to/save/file
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Xen 4.6 sees the introduction of Migration v2. There is no change for
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people using `xl`, although the `libxl` API has had an extension.
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# Technical details
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Migration is formed of several layers. `libxc` is responsible for the
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contents of the VM (ram, vcpus, etc) and the live migration loop, while
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`libxl` is responsible for items such as emulator state.
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The format of the migration v2 stream is specified in two documents, and
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is architecture neutral. Compatibility with legacy streams is
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maintained via the `convert-legacy-stream` script which transforms a
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legacy stream into a migration v2 stream.
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* Documents
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* `docs/specs/libxc-migration-stream.pandoc`
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* `docs/specs/libxl-migration-stream.pandoc`
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* `libxc`
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* `tools/libxc/xc_sr_*.[hc]`
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* `libxl`
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* `tools/libxl/libxl_stream_{read,write}.c`
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* `tools/libxl/libxl_convert_callout.c`
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* Scripts
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* `tools/python/xen/migration/*.py`
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* `tools/python/scripts/convert-legacy-stream`
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* `tools/python/scripts/verify-stream-v2`
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## libxl
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With migration v2 support, LIBXL\_HAVE\_SRM\_V2 and LIBXL\_HAVE\_SRM\_V1
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are introduced to indicate support. `domain_restore_params` gains a new
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parameter, `stream_version`, which is used to distinguish between legacy and
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v2 migration streams, and hence whether legacy conversion is required.
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# Limitations
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Hypervisor logdirty support is incompatible with hardware passthrough,
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as IOMMU faults cannot be used to track writes.
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While not a bug in migration specifically, VMs are very sensitive to
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changes in cpuid information, and cpuid levelling support currently has
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its issues. Extreme care should be taken when migrating VMs between
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non-identical CPUs until the cpuid levelling improvements are complete.
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# Testing
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Changes in libxc should be tested with every guest type (32bit PV, 64bit
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PV, HVM), while changes in libxl should test HVM guests with both
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qemu-traditional and qemu-upstream.
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In general, testing can be done on a single host using `xl
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save/restore` or `xl migrate $VM localhost`.
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Any changes to the conversion script should be tested in all upgrade
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scenarios, which will involve starting with VMs from Xen 4.5
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# Areas for improvement
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* Arm support
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* Linear P2M support for x86 PV
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* Live looping parameters
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# Known issues
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* x86 HVM guest physmap operations (not reflected in logdirty bitmap)
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* x86 HVM with PoD pages (attempts to map cause PoD allocations)
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* x86 HVM with nested-virt (no relevant information included in the
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stream)
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* x86 PV ballooning (P2M marked dirty, target frame not marked)
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* x86 PV P2M structure changes (not noticed, stale mappings used)
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# References
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Xen Developer Summit 2015 Presentation
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[video](https://www.youtube.com/watch?v=RwiDeG21lrc) and
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[slides](http://events.linuxfoundation.org/sites/events/files/slides/migv2.pdf)
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for Migration v2
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# History
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------------------------------------------------------------------------
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Date Revision Version Notes
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---------- -------- -------- -------------------------------------------
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2015-10-24 1 Xen 4.6 Document written
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---------- -------- -------- -------------------------------------------
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