Why is the capacity displayed differently on the server when configuring the volume?

Question

Hi,

I configured 100000 GB vol1 on FAS8040.

However, when the server checks the volume, the visible capacity is 86.7 Tb.

Can you see why the capacity looks different?

Below is the volume information.

FAS8040*> df -g
Filesystem                        total                        used                        avail                                 capacity                                    Mounted on
/vol/vol0/                        285GB                        14GB                   270GB                                         5%                                          /vol/vol0/
/vol/vol0/.snapshot       15GB                        0GB                        14GB                                        2%                        /vol/vol0/.snapshot
/vol/vol1/                  100000GB               91607GB                  8392GB                                        92%                                           /vol/vol1/
snap reserve                        0GB                        0GB                           0GB                                         0%                                             /vol/vol1/..
/vol/vol2/                      10000GB                     11GB                  9988GB                                         0%                                             /vol/vol2/

snap reserve                        0GB                        0GB                           0GB                                         0%                                             /vol/vol2/..

Please answer my question...

Thank you.

paul_stejskal · Accepted Answer

Which protocol? That makes a difference. If it's a LUN, you will always be looking at the file system the initiator puts on the LUN. When you look at ONTAP, you look at the WAFL file system in the FlexVol layer, or in the Aggregate. Those are 3 different things and each one will show a completely different amount of free and total space. There is typically a discrepancy between an initiator file system (NTFS, EXT4, VMFS, etc.) and a FlexVol or even the LUN size.

The reason why a LUN shows different is because it is a blob, and data may have been overwritten but not sent to be zeroed from the initiator (SCSI_UNMAP or TRIM). Rather than freeing the blocks, the LUN file in ONTAP just overwrites old blocks with new blocks as they are written if not in use anymore. The only way to get it to mostly match up is some kind of Space Reclamation or doing a zeroing operation. This is not normally recommended due to performance overhead, and must be scheduled.

I hope this helps.

If it's a NAS volume, there are a few things as mentioned in the prior post.