The heart of a dCache system is the pool manager. When a user performs an action on a file - reading or writing - a transfer request is sent to the dCache system. The pool manager then decides how to handle this request.

If a file the user wishes to read resides on one of the storage-pools within the dCache system, it will be transferred from that pool to the user. If it resides on several pools, the file will be retrieved from the pool which is least busy. If all pools the file is stored on are busy, a new copy of the file on an idle pool will be created and this pool will answer the request.

A new copy can either be created by a pool to pool transfer (p2p) or by fetching it from a connected tertiary storage system (sometimes called HSM - hierarchical storage manager). Fetching a file from a tertiary storage system is called staging. It is also performed if the file is not present on any of the pools in the dCache system. The pool manager also has to decide on which pool the new copy will be created, i.e. staged or p2p-copied.

The behaviour of the pool manager is highly configurable. In order to exploit the full potential of the software it is essential to understand the mechanisms used and how they are configured. The pool manager is a unique cell in the domain “dCacheDomain” and consists of several sub-modules: The important ones are the pool selection unit (PSU) and the cost manager (CM).

The PSU is responsible for finding the pools which the pool manager is allowed to use for a specific transfer-request. From those the CM selects the optimal one. By telling the PSU which pools are permitted for which type of transfer-request, the administrator of the dCache system can adjust the system to any kind of scenario: Separate organizations served by separate pools, special pools for writing the data to a tertiary storage system, pools in a DMZ which serves only a certain kind of data (e.g. for the grid). The following section explains the mechanism employed by the PSU and shows how to configure it with several examples.