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ONTAP LUN has less free space than Windows disk despite space allocation being enable

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ontap-9
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san
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Applies to

  • ONTAP 9
  • Windows Server 2019

Issue

  • LUN shows less free space than NTFS volume in Windows, e.g. less than 1 TB free in ONTAP and 4 TB free in WIndows
  • Space allocation is enabled and space reserve is disabled
  • NTFS DisableDeleteNotify is disabled, i.e. TRIM/UNMAP is enabled
  • When additional data is added the LUN does not fill up
  • ONTAP is receiving UNMAP commands on the LUN:
::*> statistics show -object lun -instance /vol/vol1/lun1 -counter unmap_reqs -raw 
[...]
Counter Value 
-------------------------------- -------------------------------- 
unmap_reqs 382464

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