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

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Applies to

  • ONTAP 9
  • Data ONTAP 8
  • Data ONTAP 7

Answer

  • A NetApp Storage Controller has two buffers for accepting and logging write data.
  • The Storage Controller can only process one Consistency Point (CP) per Aggregate at a time due to this buffered writing scenario.
  • The Consistency Point process is:
    • Global - all writes flow through Consistency Points (per each aggregate)
    • Atomic - all modified data is considered dirty in RAM and must be cleaned by flushing to disk
  • 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.
    • If writes arrive faster than CPU and/or disk can process:
      • The internal limit of NVLOG data is hit
      • Write latency will increase as user write operations are not replied until a write buffer frees up
    • In this scenario, the NVLOG process is not the cause but a secondary bottleneck because of overwhelmed CPU or disk resources
  • 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.

 

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