VxFS responds with read-ahead for sequential read I/O. This results in buffered I/O. The data is prefetched and retained in buffers for the application. The data buffers are commonly referred to as VxFS buffer cache. This is the default VxFS behavior.
On the other hand, direct I/O does not buffer the data when the I/O to the underlying device is completed. This saves system resources like memory and CPU usage. Direct I/O is possible only when alignment and sizing criteria are satisfied.