Used space (space after compression) = 246GB
Primary Space Saved = 76GB
246+76GB = 322GB (pre-compression) / 246GB (post-compression) = 1.31X
The bigger question is why the discrepancy between vCenter 1.62TB vs. the reported 322GB of Nimble pre-compression capacity. I believe much of this has to do with a little publicized feature on Nimble called zero-block unmap introduced in 1.4. Essentially if the Nimble array detects zero filled blocks, the system will represent that within the filesystem index and not need to go through the process of compressing these blocks and writing that to disk. Although this is represented as capacity consumed on the vCenter side, the Nimble array will not represent these zero filled blocks as used and is not calculated as part of space saved via compression. In other words, customers going from 1.3 -> 1.4 may see a decrease in system compression (zero blocks compress really well) but the system capacity used will be the relatively similar with the advantage of being much more efficient about not having to compress\write and vice versa for these zero-filled blocks.
In short, the effective compression ratio i.e. what is reported as space used on the host/what is actually used post-compression on the Nimble array is typically far better than what is reported on primary compression ratio on the Nimble GUI.
Hope this helps.
Yes when VMware thinks the volume is full it will not be able to write any more. However due to the way we thin provision on the Nimble that saved space is never actually used and therefore doesn't take up any extra space on the Nimble. The one Exception to that is if you use Volume reserve space and have a 2TB volume with 1.2TB in use on the Nimble, 2.0TB actually used and a Volume Reserve of 2.0 TB on the Nimble you are holding extra space and not able to use that extra 800Gig of data on other volumes.
Remember that our compression is done at the block level and therefore there is no way for the application layer to know that the data was compressed. So the application layer only has a volume the size it was originally created.