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Use OngoingMigrationContext to measure and log the effective VM downtime (pause to remote resume) and the cost of each non-trivial step in the downtime window: snapshotting, sending the snapshot, and awaiting completion. This makes it straightforward to identify and reduce downtime as live migration matures. Example: ``` cloud-hypervisor: 7.703402s: <vmm> INFO:vmm/src/lib.rs:1494 -- Migration completed after 2.2s with a downtime of 298ms (goal was 300ms) cloud-hypervisor: 7.703453s: <vmm> DEBUG:vmm/src/lib.rs:1500 -- Downtime breakdown: 298ms (final_iter:269ms state:7ms send_state:19ms complete:1ms) ``` Note: downtime is measured on the source only; cross-host clock skew may cause unreliable results. # Terminology At first glance, the use of "state" and "[VM] snapshot" may seem confusing. As discussed in [0], we use "state" consistently in the migration code. On the VM side, "snapshotting" is merely the mechanism used to obtain the VM state. [0] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/7979#discussion_r3061359899 On-behalf-of: SAP philipp.schuster@sap.com Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>