Creating an XRootD storage service

Overview

An XRootD storage service is a distributed file system that is composed of individual storage services arranged in a tree of arity at most 64. There are two kinds of nodes, storage nodes and supervisor nodes. A storage node internally uses a Simple Storage Service and supports all the Simple Storage Service operations. A supervisor node is always the root of a sub-tree and can perform file searches in that sub-tree, all the while maintaining a cache of recent file search results (with a time-to-live for remembering these results).

Creating an XRootD storage service

An XRootD storage service is described by specifying the deployment of the nodes in the XRootD tree on hardware resources (i.e., hosts where individual nodes in the tree will execute and access disks available at these hosts). To this end, a wrench::XRootD::Deployment object must be created first, before the simulation is launched. An instance of a Deployment is constructed based on:

• A wrench::Simulation object;

• Optional Maps (std::map) of configurable default properties (wrench::XRootD::Property) and configurable default message payloads (wrench::XRootD::MessagePayload).

Once the deployment object is created, it can be used to add nodes to the tree, i.e., instances of the wrench::XRootD::Node class. First a root node must be instantiated by calling the wrench::XRootD::Deployment::createRootSupervisor() method.

Once the root node has been instantiated, it can be used to build the rest of XRootD tree. Some nodes in the trees are supervisors, i.e., they know about all other nodes in the subtree of which they are the root and can direct searches for files down this subtree. Creating a new supervisor node in the tree is simply done by calling the wrench::XRootD::Node::addChildSupervisor() method on the node that will be the new node’s parent.

The other kind of node is a storage node, which can store files. A storage node has an underlying Simple Storage Service that stores the files. Creating a storage node is done by calling the wrench::XRootD::Node::addChildStorageServer() on the node that will be the new node’s parent.

The example below creates a small XRootD deployment of 3 nodes, a root on host Root, a supervisor node on host Super, and a Storage node on Storage. The nodes are arranged in a tree of arity 1 as follows Root->Super->Storage. The XRootD deployment is configured to simulate all underlying communications involved during a search (the REDUCED_SIMULATION property). The cache lifetime is at most to 1 hour (the CACHE_MAX_LIFETIME property). The cache is where a supervisor keeps the locations of files that it has previous found via searches. The storage node running on Storage is created with parameters similar to that used to create a SimpleStorageService instance. In this example, it has mountpoint /, can support up to 8 concurrent data connections, and the size of the control message that is sent to it to request a file read is 2KiB. Finally, in this example, a copy of file someFile is created ab initio on the storage node.

wrench::XRootD::Deployment xrootd_deployment(simulation,
                                                  {{wrench::XRootD::Property::CACHE_MAX_LIFETIME, "3600"},
                                                   {wrench::XRootD::Property::REDUCED_SIMULATION, "false"}},
                                                  {});
auto root = xrootd_deployment.createRootSupervisor("Root");
auto super = root->addChildSupervisor("Super");
auto storage = super->addChildStorageServer(
    "Storage", "/",
    {},
    {},
    {{wrench::SimpleStorageServiceProperty::MAX_NUM_CONCURRENT_DATA_CONNECTIONS, "8"}},
    {{wrench::SimpleStorageServiceMessagePayload::FILE_READ_REQUEST_MESSAGE_PAYLOAD, 2048}});

storage->createFile(someFile);

See the documentation of wrench::XRootD::Property and wrench::XRootD::MessagePayload for all possible configuration options.

See the example simulator in the examples/action_api/XRootD directory for a more complex XRootD deployment.