It seems like Windows 8's
defrag command has some new options, including:
/KPerform slab consolidation on the specified volumes.
Does anyone know what this means in English?
This PDF seems to have an explanation of this, along with new NTFS features.
I couldn't find anything specifically explaining what this means in the context of Windows 8's defragmenter. But "slab consolidation" generally refers to moving objects so that objects that round up to the same allocation size are placed together.
The benefit of doing this is usually pretty minimal. But it does tend to reduce the average seek time when a large number of small objects are accessed.