Table of Contents
The heart of a dCache system is the
poolmanager. When a user
performs an action on a file - reading or writing - a
transfer request is sent to the dCache
poolmanager then decides how to handle this
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 has to decide on which pool the new copy will be created, i.e. staged or p2p-copied.
The behaviour of the
poolmanager service 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
poolmanager service creates the
PoolManager cell, which is a unique cell in dCache and
consists of several sub-modules: The important ones are the
pool selection unit (PSU)
and the cost manager (CM).
poolmanager can be configured by either directly editing
or via the Admin
Interface. Changes made via the Admin Interface will be
saved in the file
/opt/d-cache/config/PoolManager.conf by the
save command. This file will be parsed,
whenever the dCache starts up. It is a simple text file
containing the corresponding Admin Interface commands. It can
therefore also be edited before the system is started. It can also
be loaded into a running system with the reload
command. In this chapter we will describe this file.
The PSU is responsible for finding the pool which will be used for a specific transfer-request based on the information from the CM. 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.