Introduction to DVS

Describes how Cray Data Virtualization Service (DVS) is essential for distributing images and configuration data in the new Cray management system.

Cray Data Virtualization Service (DVS) is a distributed network service that projects local file systems resident on I/O nodes or remote file servers to compute and service nodes within the Cray system. Projecting is simply the process of making a file system available on nodes where it does not physically reside. DVS-specific configuration settings enable clients (compute nodes) to access a file system projected by DVS servers. Thus, Cray DVS, while not a file system, represents a software layer that provides scalable transport for file system services. See the mount(8) and dvs(5) man pages for more information.

DVS Use Cases

DVS plays an essential role in the new Cray management system (CMS) paradigm.

DVS is tightly coupled with the Simple Shares service.
During system boot, Netroot is mounted as a DVS mount.
DVS is used by the cray_image_binding service to distribute the Cray programming environment (PE) to compute and login nodes.

Cray automatically enables DVS and configures it with the settings necessary to achieve this functionality. Sites are not required to configure DVS further, which is why this service is level basic rather than required.

This figure illustrates how the system uses DVS to distribute PE images to compute and login nodes. The PE image is stored on the SMW and a copy is pushed out to the boot node and stored on its local storage. The boot node shares the PE image with the DVS node using NFS, and then the DVS node projects it to compute and login nodes using DVS.

Figure: PE Distribution to Compute and Login Nodes

DVS needs further configuration only if a site plans to use it to project external file systems to nodes within the Cray system. The cray_dvs service configuration parameters enable system administrators to provide their users with client mounts that can be tuned for high performance in a variety of use cases. When projecting external file systems, DVS provides I/O performance and scalability to a large number of nodes, far beyond the typical number of clients supported by a single NFS server. Operating system noise and impact on compute node memory resources are both minimized in the Cray DVS configuration. Cray DVS uses the Linux-supplied virtual file system (VFS) interface to process file system access operations. This allows DVS to project any POSIX-compliant file system. Cray has extensively tested DVS with NFS and General Parallel File System (now Spectrum Scale).

Figure: Cray DVS Projection of External File Systems

When DVS is used to project external file systems, an administrator's view of Cray DVS looks like this.