H5O_FLUSH causes all buffers associated with an object to be immediately flushed to disk without removing the data from the cache.
object_id can be any named object associated with an HDF5 file including a dataset, a group, or a committed datatype.
HDF5 does not possess full control over buffering. H5O_FLUSH flushes the internal HDF5 buffers and then asks the operating system (the OS) to flush the system buffers for the open files. After that, the OS is responsible for ensuring that the data is actually flushed to disk.