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Why is the data size of Amazon S3 larger than the logical size of the volume when using Fabric Pool?

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cloud-volumes-ontap-cvo
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core
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Applies to

  • Cloud Volume ONTAP (CVO)
  • Amazon Web Services (AWS)
  • Fabric Pool

Answer

This is by the design of Object Deletion and Defragmentation.

  • Storage efficiencies such as compression, deduplication, and compaction are preserved when moving data to the cloud tier, reducing object storage and transport costs.
  • But Fabric Pool does not delete blocks from attached object stores. Instead, Fabric Pool deletes entire objects after a certain percentage of the blocks in the object are no longer referenced by ONTAP.

Example: 

There are 1,024 4KB blocks in a 4MB object tiered to Amazon S3. 
Defragmentation and deletion do not occur until less than 205 4KB blocks (20% of 1,024) are being referenced by ONTAP.
When enough (1,024) blocks have zero references, their original 4MB objects are deleted, and a new object is created.

Additional Information

This percentage, the unreclaimed space threshold, can be customized, the default settings are 20% for Amazon S3.