dCache is a distributed storage solution. It organises storage across computers so the combined storage can be used without the end-users being aware of on precisely which computer their data is stored; end-users see simply a large amount of storage.

Because end-users need not know on which computer their data is stored, their data can be migrated from one computer to another without any interruption of service. This allows dCache storage computers to be taken out of service or additional machines (with additional storage) to be added without interrupting the service the end-users enjoy.

dCache supports requesting data from a tertiary storage system. A tertiary storage system typically uses a robotic tape system, where data is stored on a tape from a library of available tapes, which must be loaded and unloaded using a tape robot. Tertiary storage systems typically have a higher initial cost, but can be extended cheaply by added additional tapes. This results in tertiary storage systems being popular where large amounts of data must be read.

dCache also provides many transfer protocols (allowing users to read and write to data). These have a modular deployment, allowing dCache to support expanded capacity by providing additional front-end machines.

Another performance feature of dCache is hot-spot data migration. In this process, dCache will detect when a few file are being requested very often. If this happens, dCache can make duplicate copies of the popular files on other computers. This allows the load to be spread across multiple machines, so increasing throughput.

The flow of data within dCache can also be carefully controlled. This is especially important for large sites as chaotic movement of data may lead to suboptimal usage; instead, incoming and outgoing data can be marshaled so they use designated resources; allowing better throughput and guaranteeing end-user experience.

dCache provides a comprehensive administrative interface for configuring the dCache instance. This is described in the later sections of this book.

Figure 1.1. The dCache Layer Model

The layer model shown in Figure 1.1, “The dCache Layer Model” gives an overview of the architecture of the dCache system.