FSx ONTAP LUNs assigned to windows instances. How to see actual free space in OS


We have a 16TB iSCSI ONTAP volume assigned to a Windows instance. The total size of the File System raw storage is also 16TB. With data reduction and data teiring to capacity tier there's far more available space on the LUN than the OS reports. This is normal behavior There's about 9TB of data on the 16TB drive so the OS sees 7 TB free from the 16TB LUN. On the back ONTAP flle system there is at least 13TB free.

I understand I could make the Volume+LUN larger than the raw disks but was looking for a more modern solution so the OS can see the actual free SSD File System space on the LUN back end so our team can better manage data on the disk. I am not looking to dynamically grow the raw space automatically.

asked 4 months ago175 views
1 Answer


Thank you for writing to us.

I believe you have created an iSCSI LUN [1] of 16TB from an ONTAP Filesystem of 16TB storage capacity.

Upon mounting this LUN to windows OS, it will be presented to the initiator as a Disk with some overhead to the disk configuration, then filesystem headers once it is initialized and further the partition headers once volume is created. Then you would be writing data to the volume. iSCSI Client or initiator could only have view & control up to this level, and not the savings at volume or aggregate level from where LUN was created.

For iSCSI server side usage and available space iSCSI Target or ONTAP file system need to be referred. For the usage and free space at AWS FSx Ontap file system level we have cloudwatch metrics [2] [3]. Also you may use Ontap CLI for space utilization [4].

Hence it is not possible for the Client side OS to view the free space at SSD File System or LUN back end level.

[1] https://docs.aws.amazon.com/fsx/latest/ONTAPGuide/create-iscsi-lun.html [2] https://docs.aws.amazon.com/fsx/latest/ONTAPGuide/file-system-metrics.html [3] https://docs.aws.amazon.com/fsx/latest/ONTAPGuide/monitor-fs-storage.html [4] https://docs.netapp.com/us-en/ontap/volumes/commands-display-space-usage-reference.html

answered 4 months ago

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