"x" for to-be-determined (but the name stuck)

Large files: up to 9 ExaBytes (16TB on 32-bit systems).

Large filesystems: up to 18 ExaBytes (16TB on 32-bit systems).

Large number of inodes

Large directories

Large I/O

Parallelized access via allocation groups

Balanced tree (btree) algorithms for searching large lists

Asynchronous metadata transaction logging for quick recovery

Delayed allocation to improve data contiguity and performance

Extended attributes (such as Access Control Lists)

Extent based allocation (including unwritten extents)

Variable allocation block sizes

Direct I/O