Investigate interactions between direct I/O VFD and data sieving

When data sieving is enabled and used, direct I/O may first be performed on a potentially-unaligned sieve buffer before being copied into a potentially-aligned buffer allocated by an application; it is unclear whether this behavior is desirable. In the best case, this could have undesired I/O performance characteristics. In the worst case, this could result in I/O errors due to unaligned access.