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What is Consistency Point, and why does NetApp use it?

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Category:
ontap-9
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perf
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Applies to

  • ONTAP 9
  • Data ONTAP 8
  • Data ONTAP 7

Answer

  • A storage controller has two separate memory buffers for storing write data.
  • The size of these buffers is based on the amount of non-volatile RAM (NVRAM) in a given system.
  • This can be calculated as 1/2 the NVRAM size for stand-alone systems, 1/4 the NVRAM size for clustered systems, and 1/8 the NVRAM size for 4/8-node MetroClusters.
  • All writes into a storage controller are stored concurrently in the following locations:
    • Local memory buffer
    • Local NVRAM/NVMEM
    • Remote NVRAM (Clustered systems only)
    • Remote NVRAM of the DR partner (MetroCluster systems only)
  • As soon as the process to log to NVRAM is confirmed, the controller acknowledges the write as completed to the client machine (app, protocol?).
  • At pre-defined triggers, this buffered write data is processed from the storage controller memory through the Write Anywhere File Layout (WAFL) and RAID layers and written to disk.
  • The active file system pointers on the disk are not updated to point to the new locations until a write is completed.
  • Upon completion of a write to disk, the contents of NVRAM are cleared and made ready for the next batch of incoming write data.
    • This act of writing data to disk and updating active file system pointers is called a Consistency Point (CP).
  • During the writing phase of one memory buffer, the second memory buffer and NVRAM space ( both local and remote ) are used to store and acknowledge incoming writes.

 

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