6.1 GPFS description

GPFS is a major component of the IBM clusters offering. Its purpose is to satisfy the needs of the most demanding applications in terms of I/O throughput and resilience. This is obtained by combining the processing power and disk bandwidth of selected nodes and making them available to the whole cluster.

There are other file systems that can be used to provide shared and consistent access to files inside a cluster. Network File System (NFS) is still very much in use. However, if an NFS can be mounted on all the nodes in a cluster, the files are still served by a single node, and this quickly becomes a bottleneck. This single server architecture does not scale well when the number of client nodes increases .

The open source community provides Parallel Virtual File System (PVFS ^[1] ), which uses concepts similar to GPFS. It can use any number of server nodes, each node using its own disks and disk adapters. It is known to work on pSeries Linux clusters, but lacks the high availability features of GPFS.

^[1] http://www.parl.clemson.edu/pvfs/

Here we describe the GPFS architecture and components and how they operate . However, this description is by no means exhaustive and the interested reader should refer to the following documentation for more information:

• Linux Clustering with CSM and GPFS , SG24-6601

• The complete GPFS documentation can be viewed at:

  http://www.ibm.com/servers/eserver/clusters/software/gpfs.html

6.1.1 Principles of operation

GPFS version 2.2 supports two modes of operation, depending on the type of attachment of the storage devices to the nodes: direct-attached disks, and Network Shared Disks. We describe these as follows :

• Direct-attached disks

  In direct attachment mode, all nodes see all the disks allocated to GPFS, generally through a SAN. In this mode, the path for the file data between the GPFS nodes is through the fiber channel attachments. All the nodes participating to the GPFS cluster must have an attachment to the SAN.

• Network Shared Disk

  A subset of the GPFS nodes are designated as the GPFS servers. They "export" their locally attached disks, through a network disk abstraction called Network Shared Disk (NSD), to the other cluster nodes.

  This time, the file data flows between the GPFS server nodes and the client nodes over a TCP/IP network connection. The constraint here to have a fast network connection between the nodes. The network of choice in this mode of operation is Gigabit Ethernet.

Figure 6-1 illustrates the direct-attached disks mode of operation.

Figure 6-1. GPFS with direct-attached disks

The Network Shared Disks mode is shown in Figure 6-2 on page 282.

Figure 6-2. GPFS with Network Shared Disks

GPFS is implemented at the VFS level so that applications can use it transparently just like any other file system. GPFS also provides an API for applications that would like to benefit from some of the features that are not accessible through the UNIX file system interface.

On AIX pSeries clusters, IBM Parallel Environment 4.1 implements the I/O part of the MPI 2 standard with GPFS, providing a very high performance parallel I/O environment for large parallel applications.

6.1.2 Terminology

GPFS defines various entities, as described here:

GPFS cluster

GPFS cluster identifies the collection of nodes that you intend to run GPFS on. GPFS clusters can be of different types, but the only type supported on Linux is the loose cluster type or "lc". AIX GPFS clusters can be of other types: hacmp in HACMP environments, or sp if defined on AIX clusters equipped with SP Switch or SP Switch 2.

With the lc type, GPFS clusters can be interoperable between AIX and Linux on xSeries. Nodes in a GPFS cluster must use the same communication adapters. In the case of Linux on pSeries, only Ethernet is supported.

GPFS nodeset

A nodeset is a subset of the GPFS cluster, and this is where the file systems are defined. A node can only belong to one nodeset at a time. It is really where all the configuration takes place.

Examples of attributes that are private to a GPFS nodeset are quorums, and the maximum size of the caching attributes of the file systems, and the communication protocol to use between the participating nodes.

Failure group

A failure group identifies the set of disks that have a common point of failure. For example, two disks on the same adapter on the same node are part of the same failure group.

This information is used by GPFS to decide on data placement in order to improve data availability. Indeed, there is no point to having two replicas on the same data in the same failure group, as they would both become unavailable in case of a failure.

6.1.3 Components

GPFS consists of the following components:

• Administrative commands

  These reside under /usr/lpp/mmfs/bin. You should update your PATH to include this directory.

• The GPFS daemon(mmfsd)

  This is a multi-threaded daemon that runs on all the nodes in the nodeset. It is responsible for the following:

  - Performing all I/O operations on files

  - Managing directories

  - Maintaining data integrity

  - Satisfying disk space allocation requests

• The kernel module (mmfs)

  This is used for interfacing with the VFS layer. System calls to access files in a GPFS file system will be handed by the Linux kernel to this module.

• The portability layer module (mmfslinux)

  This open source module has to be compiled during GPFS installation. It provides a portable interface between the kernel and the rest of GPFS, which is provided as binaries only.

• tracedev

  This module provides a trace facility for GPFS. Under normal circumstances, tracing is disabled, and it only needs to be enables in the course of troubleshooting under the guidance of IBM support personnel.

  However, the tracedev module will be loaded every time at GPFS startup. It also ships as source code and needs to be built together with mmfslinux (the same set of makefiles build both).

6.1.4 Advantages

As a general purpose file system, GPFS has several unparalleled features that we detail now.

I/O performance

By aggregating the individual I/O performance and processing power of many cluster nodes, GPFS can dramatically improve the disk throughput of serial and parallel applications. A sound design should make sure that you match the performance characteristics of all the components (nodes, disks, network or fiber adapters) to avoid bottlenecks. GPFS is at its best for large sequential I/O patterns, and it is the file system of choice for distributed applications that need to operate concurrent read and write operations on files.

High availability

When properly configured, GPFS provides a highly resilient file system. It can recover from failures at the disk, adapter, or node level. Node failure recovery is facilitated with three components of RSCT:

• System Resource Controller (SRC) to start, stop, and interact with RSCT daemons

• Topology Services, which constantly monitors the status of nodes and network adapters in the cluster

• Group Services, which provides coordination between processes executing on distributed nodes by maintaining group-wide information and informing members of the group of events affecting the group.

With data replication enabled, not only can the structure and operation of the file system be continuous, but the data can also be made highly available.

Flexibility

Disks and nodes can be added on the fly to a GPFS nodeset. When it becomes more convenient from an operational point of view, the data can then be rebalanced between the various disks and nodes.