Messaging Solutions in a Linux Environment

42 Messaging Solutions in a Linux Environment

Figure 2-4 Linux Virtual Server

For more information about the project and download, please visit:

http://www.linuxvirtualserver.org/

2.3 Linux technologies for high availability

This section discusses some technologies that are relevant for making a Linux

solution scalable.

2.3.1 Journaled file systems

Journaling file systems offer much faster system recovery in the event of system

failure than nonjournaled file systems such as HPFS and ext2. Instead of relying

on boot time utilities like fsck to examine the file system’s metadata in order to

restore the file system to a consistent state, they simply read and apply a

transaction log.

The Journaled File System

The Journaled File System (JFS) for Linux is an IBM open source project

licensed under the GNU GPL. It has been developed with transaction-oriented,

high performance systems in mind. It is scalable and robust, providing a

LAN/WAN

Real Server 1

Real Server 2

Real Server 3

Load Balancer

Internet

Chapter 2. Linux systems and advanced technologies 43

log-based, byte-level file system that was designed to include fully integrated

journaling from the beginning rather than adding the journaling feature to an

existing file system. Its journaling techniques were originally developed for

databases, which log operations performed on the data records, although JFS

only implements logging of metadata. Some of its other features include:

򐂰 Dynamic disk inode allocation

JFS dynamically allocates space for disk inodes as required and frees the

space when it is no longer required. This approach eliminates the need for

reserving a fixed amount of space for disk inodes at the file system creation

time, hence it will not be necessary for users to estimate the maximum

number of files and directories on a file system.

򐂰 Variable block size

Block sizes of 512, 1024, 2048 and 4096 are supported on an individual file

system basis, allowing a user to select the appropriate size for his application

set. A smaller block size reduces file fragmentation and is more

space-efficient. On the other hand, space allocation occurs more often than if

a large block size were to be used, reducing performance.

JFS is included in most of the major distributions: Red Hat, SUSE, Debian and

Turbo Linux. For free download and more information about the JFS file system

and its features, visit:

http://www-124.ibm.com/developerworks/opensource/jfs/

General Parallel File System for Linux

GPFS is IBM’s high performance cluster file system. Its main features are:

򐂰 Scalable file system

The system is designed for clusters where the number of nodes and the file

system size can be large. It allows expansion of file system throughput and

capacity as service demands increase.

򐂰 Parallel file system

Employs multiple nodes of a cluster to store blocks of a single file. Exploits

parallel I/O to circumvent bottlenecks that might be present if a file resided on

a single node.

򐂰 Journaling file system

Logs information about operations performed on the file system metadata as

atomic transactions that can be replayed. Provides rapid restoration of file

system to consistent state after system crash.

44 Messaging Solutions in a Linux Environment

򐂰 Shared file system

Implemented so that multiple clients can concurrently access a file and

includes a sophisticated token management system to ensure data

consistency. Allows nodes running parallel applications to concurrently

access a file and thereby reduces processing time.

򐂰 High availability file system

Provides for automated recovery from events that would normally interrupt

data availability. Applications and users can continue without interruption.

򐂰 Client side data caching

Keeps records of recent I/Os in client memory. Improves performance by

reducing need for repetitive processing of requests for frequently accessed

files and also hides write latency by using "write behind".

򐂰 Large Blocksize options

Blocksizes of 16K, 64K, 256K, 512K or 1024K can be defined. Provides for

efficient use of disk bandwidth and disk space, particularly important for RAID

devices.

򐂰 Access pattern recognition and deep prefetch

Detects access pattern as sequential, random, fuzzy sequential or strided,

and prefetches striped data in parallel. Enables high throughput to be

sustained, by overlapping disk latencies.

򐂰 Data protection

Both metadata and user data can be replicated in the GPFS installation.

Provides protection from loss of data and metadata.

GPFS supports two separate models of disk attachment and a combination of

the two is also allowed. This is very useful if, for example, you wanted to create a

disaster recovery site by duplicating your data across two physical sites.

Directly attached model

All the nodes are directly connected to the storage with a SAN switch, for

example. See Figure 2-5 on page 45.