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What are the benefits of Consistency Points versus Direct Writes?

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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

The following characteristics are some of the primary benefits of using Consistency Points instead of passing writes directly to disk:

  • WAFL file system integrity:
    • Even during a power loss, the WAFL file system remains in a consistent state.
      • All operations acknowledged by the controller to the clients are confirmed consistent and are preserved intact.
      • This allows for rapid boot up no matter how the node was taken down since WAFL file system consistency checking is not required.
  • Increase in client write performance:
    • Since memory is significantly faster than spinning media, the end client receives faster acknowledgment of its writes and can continue processing the next write request.
      • A performance gain is also attained on the disk subsystem since the write operations to disk can be grouped together into larger sequential writes instead of many smaller random writes.
  • Optimized data layout:
    • Since the writes to disk are processed in bulk, the WAFL subsystem can allocate larger swaths of data in contiguous space on the disk.
      • This minimizes the possibility of data layout randomization and thus prevents the need for data reallocation.
  • Decreased requirement for non-volatile RAM (NVRAM) / Memory buffer size:
    • This is not obvious at first; however, logging write requests as they arrive consumes a considerably smaller amount of memory than would be required if the caching was done after processing by Write Anywhere File Layout (WAFL) and RAID layers.
    • This also prevents a failure of NVRAM from resulting in a damaged filesystem that normal file system consistency checks cannot detect nor correct.

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