Estou surpreso que você tenha uma configuração tão grande. Você construiu este array? Isso é potencialmente um mau arranjo para o desempenho devido ao design do pool.
De qualquer forma, a página zpool man explica isso. zfs list mostrará seu espaço utilizável. A lista zpool mostra o espaço de paridade como espaço de armazenamento.
used Amount of storage space used within the pool.
The space usage properties report actual physical space available to the storage pool. The physical
space can be different from the total amount of space that any contained datasets can actually use.
The amount of space used in a raidz configuration depends on the characteristics of the data being
written. In addition, ZFS reserves some space for internal accounting that the zfs(8) command takes
into account, but the zpool command does not. For non-full pools of a reasonable size, these effects
should be invisible. For small pools, or pools that are close to being completely full, these dis-
crepancies may become more noticeable.