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AIX EXTRA: Advanced File System Support in AIX


IBM's AIX operating system comes bundled with multiple file system options for managing data, including journal file system (JFS), extended journal file system (JFS2), CD file system (CDRFS) and DVD file system (UDFS), as well as the Veritas JFS2, VxFS. Let's examine some of the attributes, features and uses for JFS and JFS2, included with AIX 5L.

Utilizing techniques developed for data integrity in a database environment, journal file systems use logging techniques to capture information about the commitment of data to the file system structures. This information is used to cleanly recover from the unplanned file system shutdown that happens when the server crashes or loses power.

Without this logging feature, a file system must review all of its data structures for integrity and consistency. In some of the very large file systems common in data processing environments, this could take hours--even days. During this exhaustive file system check (FSCK), the information contained in the file system and the IT service that utilizes that data are unavailable. The result is extended service outages following the unplanned system down. Traditional Berkley Fast File Systems--such as the Hewlett Packard HFS--suffer from the extended file system checks.

Journal file systems such as the IBM JFS and JFS2 use advanced database-logging techniques--such as log files to track write transactions during their commitment--to avoid such outages. During an unexpected system interruption, the file system check utility automatically replays the logs (using logredo) as the system is rebooted and before the file system is remounted, thus helping to ensure a clean file system. The result is a file system integrity check--for very large journal file systems--that lasts seconds rather than hours.

AIX supports traditional JFS and JFS2. The latter was introduced with AIX 5L to support very large file systems and file systems that support very large files. These two file systems have some fundamental differences, as shown Table 1.

JFS2 was designed to improve on JFS performance and capacity and leverage the benefits of the 64-bit AIX. As a result, JFS2 provides some significant benefits for the size of the file system and the files supported within the file systems, which can be particularly important in large database or very large file system environments common in scientific computing. The term "extended" implies that JFS2 simply expands upon JFS, but Table 1 reveals that important JFS attributes--such as quota management, data-file compression and directory hard links—were sacrificed in order to realize the size and performance benefits.

 

Jim Rice is the Principal IT Architecture Consultant for MSI Systems Integrators. Jim can be reached at jrice@msiinet.com.


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