Q. Should I defragment my volumes on Storage Spaces Direct virtual disks?
A. Behind the scenes Storage Spaces Direct spreads data over multiple disks on multiple nodes with large numbers of disks being used to store the actual data. This means that just like with a SAN, trying to defragment data so its contiguous does not really bring benefit since the data is split over many disks and the typical seek penalty where data is not contiguous is not valid because of how data is distributed over spindles and likely SSDs which don't require defragmentation anyway. Remember also that Storage Spaces Direct uses tiering and by performing a defragmentation blocks would be touched which may interfere with the tiering logic degrading overall performance.