The Bad News:
I/O operations are generally regarded as inhibitors to parallelism.
I/O operations require orders of magnitude more time than memory operations.
Parallel I/O systems may be immature or not available for all platforms.
In an environment where all tasks see the same file space, write operations can result in file overwriting.
Read operations can be affected by the file server's ability to handle multiple read requests at the same time.
I/O that must be conducted over the network (NFS, non-local) can cause severe bottlenecks and even crash file servers.