[x] For threads: while pread() and pwrite() are thread-safe, other internal data structure such as known ranges is not thread safe.
[x] For processes: POSIX doesn't guarantee consistent read and write against single file by multiple processes; and little is also supported in Linux. Same technique as we do in pfio.cache.MultiprocessFileCache can be applied to the class SparseFileCache().
SparseFileCache()
.