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Why is Volume write throughput much higher than Workload write throughput when SCSI WRITE SAME is present

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Applies to

  • ONTAP 9
  • AIQ PAS
  • SCSI WRITE SAME 

Answer

This is the expected behavior. 

  • SCSI WRITE SAME is a SCSI operation that tells the storage to write a certain pattern a number of times to consecutive blocks.
  • In most cases, SCSI WRITE SAME  is used to write all zeros so the client can initialize the data on block devices. 
  • Instead of issuing a large amount of zeros, the client only has to issue a few hundred or thousand small WRITE SAME operations, that's why this payload will NOT be charged at the Workload or LUN layer. 
  • ONTAP will convert these WRITE SAME operations to a lot more WAFL WRITE messages, that's why the payload/throughput is observed with Volume layer. 

 

Additional Information

SCSI Copy Offload works in a similar way and might also lead to a discrepancy in throughput between the Workload layer and the Volume layer. 

  • ONTAP has per-volume statistics to track down the Copy Offload opeations. 
  • Copy Offload will cause a discrepancy for both the Write Ops/Throughput and the Read Ops/Throughput, depending on it's the Copy Offload source or destination. 

 

 

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