Check free memory available on USS including extents availavle

[skurnevich]

The df command displays space for already allocated extents only, quote from man df:

  *  Space available and total space.

     The total space reported is the space in the already allocated extents
     (primary and any already allocated secondary extents) of the data set that
     holds this file system. Therefore, the total space might increase as new
     extents are allocated. 

Review if the zfsadm aggrinfo or some other command may provide more accurate data