With the prevalence of Internet application and data-intensive computing, there are
many new application systems in cloud computing environment. These systems are mainly
characterized by [1-3]: (1) the enormous files stored in the system, some even reach
trillions level, and it still increase rapidly; (2) The user number and daily access are quire
enormous, reaching billions level. For example, the number of pictures currently stored in
Facebook [4] is more than 140 billion, and both the number of users and daily access are
over one billion. Such massive files and accesses may result in that the expandability and
high performance will be the bottleneck of the efficient application of cloud computing,
and meanwhile, the efficient management of metadata is a key technology that can break
through this bottleneck.