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What is the Back-to-Back (B2B) Consistency Point Scenario?

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ontap-9
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perf
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Applies to

  • ONTAP 9
  • Data ONTAP 8
  • Data ONTAP 7

Answer

For more information, see this KB: Write Performance Impacted by Back to Back Consistency Points

  • A NetApp Storage Controller has two buffers for accepting and logging write data.
  • The Storage Controller can only process one Consistency Point (CP) perf Aggregate at a time due to this buffered writing scenario.
  • Under certain circumstances, while one CP is being processed and written to disk, the second memory buffer can reach a watermark that triggers a new CP prior to the previous CP being completed.
  • Since the Consistency Point process is global per aggregate (meaning ALL writes for that aggregate flow through this mechanism), and atomic (meaning all changed data that is part of the CP must be committed to disk in order to complete it), a Storage Controller in this situation must momentarily delay acknowledging ALL incoming write data requests until the previous CP is completed and the corresponding Non-Volatile RAM (NVRAM) and local memory buffers are cleared.
  • In most instances of this specific scenario, the time at which the storage controller must pause incoming write requests is measured in milliseconds, and the environment is not significantly impacted.
    • However, on storage controllers that fall into one or both of the categories below, the impact on overall performance might be undesirable.

Additional Information

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