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SUSE Linux Enterprise Server (SLES 10) Installation and Administration
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14.1 Overview of OCFS2

Oracle Cluster File System 2 (OCFS2) is a general-purpose journaling file system that is fully integrated in the Linux 2.6 kernel and later. OCFS2 allows you to store application binary files, data files, and databases on devices in a SAN. All nodes in a cluster have concurrent read and write access to the file system. A distributed lock manager helps prevent file access conflicts. OCFS2 supports up to 32,000 subdirectories and millions of files in each directory. The O2CB cluster service (a driver) runs on each node to manage the cluster.

14.1.1 Features and Benefits

In August 2005, OCFS2 was added to SUSE Linux Enterprise Server 9 to support Oracle Real Application Cluster (RAC) databases and Oracle Home (its application files). In SUSE Linux Enterprise Server 10 and later, OCFS2 can be used for any of the following storage solutions:

  • Oracle RAC and other databases

  • General applications and workloads

  • XEN image store in a cluster

    XEN virtual machines and virtual servers can be stored on OCFS2 volumes that are mounted by cluster servers to provide quick and easy portability of XEN virtual machines between servers.

  • LAMP (Linux, Apache, MySQL, and PHP | Pearl | Python) stacks

In addition, it is fully integrated with Heartbeat 2.

As a high-performance, symmetric, parallel cluster file system, OCFS2 supports the following functions:

  • An application’s files are available to all nodes in the cluster. Users simply install it once on an OCFS2 volume in the cluster.

  • All nodes can concurrently read and write directly to storage via the standard file system interface, enabling easy management of applications that run across a cluster.

  • File access is coordinated through the Distributed Lock Manager (DLM).

    DLM control is good for most cases, but an application’s design might limit scalability if it contends with the DLM to coordinate file access.

  • Storage backup functionality is available on all back-end storage. An image of the shared application files can be easily created, which can help provide effective disaster recovery.

OCFS2 also provides the following capabilities:

  • Metadata caching

  • Metadata journaling

  • Cross-node file data consistency

  • A GTK GUI-based administration via the ocfs2console utility

  • Operation as a shared-root file system

  • Support for multiple-block sizes (each volume can have a different block size) up to 4 KB, for a maximum volume size of 16 TB

  • Support for up to 255 cluster nodes

  • Context-dependent symbolic link (CDSL) support for node-specific local files

  • Asynchronous and direct I/O support for database files for improved database performance

14.1.2 O2CB Cluster Service

The O2CB cluster service is a set of modules and in-memory file systems that are required to manage OCFS2 services and volumes. You can enable these modules to be loaded and mounted system boot. For instructions, see Configuring OCFS2 Services.

Table 14-1 O2CB Cluster Service Stack

Service

Description

Node Manager (NM)

Keeps track of all the nodes in the /etc/ocfs2/cluster.conf file

Heartbeat (HB)

Issues up/down notifications when nodes join or leave the cluster

TCP

Handles communications between the nodes with the TCP protocol

Distributed Lock Manager (DLM)

Keeps track of all locks and their owners and status

CONFIGFS

User space configuration file system. For details, see In-Memory File Systems.

DLMFS

User space interface to the kernel space DLM. For details, see In-Memory File Systems.

14.1.3 Disk Heartbeat

OCFS2 requires the nodes to be alive on the network. The O2CB cluster service sends regular keepalive packets to ensure that they are. It uses a private interconnect between nodes instead of the LAN to avoid network delays that might be interpreted as a node disappearing and thus, lead to a node’s self-fencing.

The OC2B cluster service communicates the node status via a disk heartbeat. The heartbeat system file resides on the SAN, where it is available to all nodes in the cluster. The block assignments in the file correspond sequentially to each node’s slot assignment.

Each node reads the file and writes to its assigned block in the file at two-second intervals. Changes to a node’s time stamp indicates the node is alive. A node is dead if it does not write to the heartbeat file for a specified number of sequential intervals, called the heartbeat threshold. Even if only a single node is alive, the O2CB cluster service must perform this check, because another node could be added dynamically at any time.

You can modify the disk heartbeat threshold in the /etc/sysconfig/o2cb file, using the O2CB_HEARTBEAT_THRESHOLD parameter. The wait time is calculated as follows: 

(O2CB_HEARTBEAT_THRESHOLD value - 1) * 2 = threshold in seconds

For example, if the O2CB_HEARTBEAT_THRESHOLD value is set at the default value of 7, the wait time is 12 seconds ((7 - 1) * 2 = 12).

14.1.4 In-Memory File Systems

OCFS2 uses two in-memory file systems for communications:

Table 14-2 In-Memory File Systems Used by OCFS2

In-Memory File System

Description

Mount Point

configfs

Communicates the list of nodes in the cluster to the in-kernel node manager, and communicates the resource used for the heartbeat to the in-kernel heartbeat thread 

/config

ocfs2_dlmfs

Communicates locking and unlocking for clusterwide locks on resources to the in-kernel distributed lock manager that keeps track of all locks and their owners and status 

/dlm

14.1.5 Management Utilities and Commands

OCFS2 stores parameter files specific to the node on the node. The cluster configuration file (/etc/ocfs2/cluster.conf) resides on each node assigned to the cluster.

The ocfs2console utility is a GTK GUI-based interface for managing the configuration of the OCFS2 services in the cluster. Use this utility to set up and save the /etc/ocfs2/cluster.conf file to all member nodes of the cluster. In addition, you can use it to format, tune, mount, and umount OCFS2 volumes.

Additional OCFS2 utilities are described in the following table. For information about syntax for these commands, see their man pages.

Table 14-3 OCFS2 Utilities

OCFS2 Utility

Description

debugfs.ocfs2

Examines the state of the OCFS file system for the purpose of debugging.

fsck.ocfs2

Checks the file system for errors and optionally repairs errors.

mkfs.ocfs2

Creates an OCFS2 file system on a device, usually a partition on a shared physical or logical disk. This tool requires the O2CB cluster service to be up.

mounted.ocfs2

Detects and lists all OCFS2 volumes on a clustered system. Detects and lists all nodes on the system that have mounted an OCFS2 device or lists all OCFS2 devices.

ocfs2cdsl

Creates a context-dependent symbolic link (CDSL) for a specified filename (file or directory) for a node. A CDSL filename has its own image for a specific node, but has a common name in the OCFS2.

tune.ocfs2

Changes OCFS2 file system parameters, including the volume label, number of node slots, journal size for all node slots, and volume size.

Use the following commands to manage O2CB services. For more information about the o2cb command syntax, see its man page.

Table 14-4 O2CB Commands

Command

Description

/etc/init.d/o2cb status

Reports whether the o2cb services are loaded and mounted

/etc/init.d/o2cb load

Loads the O2CB modules and in-memory file systems

/etc/init.d/o2cb online ocfs2

Onlines the cluster named ocfs2

At least one node in the cluster must be active for the cluster to be online.

/etc/init.d/o2cb offline ocfs2

Offlines the cluster named ocfs2

/etc/init.d/o2cb unload

Unloads the O2CB modules and in-memory file systems

/etc/init.d/o2cb start ocfs2

If the cluster is set up to load on boot, starts the cluster named ocfs2 by loading o2cb and onlining the cluster

At least one node in the cluster must be active for the cluster to be online.

/etc/init.d/o2cb stop ocfs2

If the cluster is set up to load on boot, stops the cluster named ocfs2 by offlining the cluster and unloading the O2CB modules and in-memory file systems

14.1.6 OCFS2 Packages

The OCFS2 kernel module (ocfs2) is installed automatically in SUSE Linux Enterprise Server 10 and later. To use OCFS2, use YaST (or the command line if you prefer) to install the ocfs2-tools and ocfs2console packages on each node in the cluster.

  1. Log in as the root user or equivalent, then open the YaST Control Center.

  2. Select Software Software Management.

  3. In the Search field, enter 

    ocfs2

    The software packages ocfs2-tools and ocfs2console should be listed in the right panel. If they are selected, the packages are already installed.

  4. If you need to install the packages, select them, then click Install and follow the on-screen instructions.

SUSE Linux Enterprise Server (SLES 10) Installation and Administration
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  Published Courtesy of Novell, Inc. Design by Interspire