The VMFS5 file system uses a unified block size of 1MB, whereas VMFS3 used different block sizes depending on the maximum size of the files, i.e. virtual disks stored on the datastore. However, when you upgrade a VMFS3 datastore to VMFS5 the block size will remain the same as it was configured for VMFS3. In order to change the block size you need to re-format the datastore.
André
