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106 lines
3.3 KiB
ReStructuredText
106 lines
3.3 KiB
ReStructuredText
QEMU Migration Phases
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=====================
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.. contents::
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QEMU supports only migration protocols 2 and 3 (1 was lacking too many
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steps). Repeating the protocol sequences from libvirt.c:
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Migration protocol v2 API Sequence
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----------------------------------
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**Src**: ``DumpXML``
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- Generate XML to pass to dst
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**Dst**: ``Prepare``
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- Get ready to accept incoming VM
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- Generate optional cookie to pass to src
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**Src**: ``Perform``
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- Start migration and wait for send completion
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- Kill off VM if successful, resume if failed
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**Dst**: ``Finish``
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- Wait for recv completion and check status
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- Kill off VM if unsuccessful
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Migration protocol v3 API Sequence
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----------------------------------
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**Src**: ``Begin``
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- Generate XML to pass to dst
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- Generate optional cookie to pass to dst
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**Dst**: ``Prepare``
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- Get ready to accept incoming VM
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- Generate optional cookie to pass to src
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**Src**: ``Perform``
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- Start migration and wait for send completion
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- Generate optional cookie to pass to dst
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**Dst**: ``Finish``
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- Wait for recv completion and check status
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- Kill off VM if failed, resume if success
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- Generate optional cookie to pass to src
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**Src**: ``Confirm``
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- Kill off VM if success, resume if failed
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QEMU Migration Locking Rules
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============================
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Migration is a complicated beast which may span across several APIs on both
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source and destination side and we need to keep the domain we are migrating in
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a consistent state during the whole process.
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To avoid anyone from changing the domain in the middle of migration we need to
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keep ``MIGRATION_OUT`` job active during migration from ``Begin`` to
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``Confirm`` on the source side and ``MIGRATION_IN`` job has to be active from
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``Prepare`` to ``Finish`` on the destination side.
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For this purpose we introduce several helper methods to deal with locking
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primitives (described in `qemu-threads <qemu-threads.html>`__) in the right way:
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* ``qemuMigrationJobStart``
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* ``qemuMigrationJobContinue``
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* ``qemuMigrationJobStartPhase``
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* ``qemuMigrationJobSetPhase``
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* ``qemuMigrationJobFinish``
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The sequence of calling ``qemuMigrationJob*`` helper methods is as follows:
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- The first API of a migration protocol (``Prepare`` or ``Perform/Begin``
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depending on migration type and version) has to start migration job and keep
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it active::
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qemuMigrationJobStart(driver, vm, VIR_JOB_MIGRATION_{IN,OUT});
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qemuMigrationJobSetPhase(vm, QEMU_MIGRATION_PHASE_*);
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...do work...
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qemuMigrationJobContinue(vm);
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- All consequent phases except for the last one have to keep the job active::
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if (!qemuMigrationJobIsActive(vm, VIR_JOB_MIGRATION_{IN,OUT}))
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return;
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qemuMigrationJobStartPhase(driver, vm, QEMU_MIGRATION_PHASE_*);
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...do work...
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qemuMigrationJobContinue(vm);
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- The last migration phase finally finishes the migration job::
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if (!qemuMigrationJobIsActive(vm, VIR_JOB_MIGRATION_{IN,OUT}))
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return;
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qemuMigrationJobStartPhase(driver, vm, QEMU_MIGRATION_PHASE_*);
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...do work...
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qemuMigrationJobFinish(vm);
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While migration job is running (i.e., after ``qemuMigrationJobStart*`` but before
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``qemuMigrationJob{Continue,Finish}``), migration phase can be advanced using::
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qemuMigrationJobSetPhase(vm, QEMU_MIGRATION_PHASE_*);
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