HCX High I/O Latency During Bulk Migration Delta Sync Cycles - VMware HCX
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 HCX High I/O Latency During Bulk Migration Delta Sync Cycles - VMware HCX

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Article ID: 453599

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Updated On:

Products

VMware HCX

Issue/Introduction

Introduction HCX Bulk Migration operations utilize a fixed 2-hour Recovery Point Objective (RPO) cycle for delta synchronization. For high-churn, large-scale workloads (e.g., Oracle databases, 16+ TB), this delta sync process can introduce significant I/O latency on the source environment. This article addresses identifying and mitigating performance degradation during these synchronization intervals.

  • Increased VMDK write latency observed during HCX delta sync cycles.
  • Performance degradation within the Guest OS (e.g., Oracle DB query latency).
  • No latency impact observed during the initial base sync.

Environment

  • VMware HCX 4.x
  • vSphere 7.0.x, 8.0.x
  • High-churn workloads (Databases, Log-heavy applications)

Cause

HCX Bulk Migration employs a fixed 2-hour RPO interval for delta synchronization. For large virtual disks with high data churn, the replication process causes periodic I/O overhead as the system calculates and transfers delta changes. This is standard behavior for the Bulk Migration workflow.

Resolution

To mitigate I/O performance impact on high-churn workloads, implement the following steps:

  1. Quiesce High-I/O Tasks: Before the migration window, suspend non-essential high-I/O tasks such as database indexing, log rotation, or automated file system scanning within the Guest OS.
  2. Monitor via Command Line: During migration, if the HCX UI appears unresponsive or stuck, verify replication progress using the ESX CLI: vim-cmd hbrsvc/vmreplica.getState <VM_ID>
  3. Utilize "Switchover Now": If the delta sync performance impact is unacceptable and the initial sync is complete, perform a "Switchover Now" to immediately trigger the cutover and end the RPO cycle.
  4. Evaluate Alternative Replication: For workloads where a 2-hour RPO creates unmanageable performance impact, evaluate the following:
    • Replication-Assisted vMotion (RAV): Offers live transfer capabilities with lower RPO impact.
    • Standalone vSphere Replication: Allows for granular RPO configuration tailored to specific application requirements.
  5. Schedule Window: Ensure maintenance windows are adequately estimated to account for lengthy data checksum and disk consolidation processes for large VMs (10+ TB).

Additional Information

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