Veritas™ File System 5.0.1 Administrator's Guide

■ File system disk layouts

Logging

A key aspect of any file system is how to recover if a system crash occurs. Earlier

methods required a time-consuming scan of the entire file system. A better solution

is the method of logging (or journaling) the metadata of files.

VxFS logs new attribute information into a reserved area of the file system,

whenever file system changes occur. The file system writes the actual data to disk

only after the write of the metadata to the log is complete. If and when a system

crash occurs, the system recovery code analyzes the metadata log and tries to

clean up only those files. Without logging, a file system check (fsck) must look

at all of the metadata.

Intent logging minimizes system downtime after abnormal shutdowns by logging

file system transactions. When the system is halted unexpectedly, this log can be

replayed and outstanding transactions completed. The check and repair time for

file systems can be reduced to a few seconds, regardless of the file system size.

By default, VxFS file systems log file transactions before they are committed to

disk, reducing time spent checking and repairing file systems after the system is

halted unexpectedly.

Extents

An extent is a contiguous area of storage in a computer file system, reserved for

a file. When starting to write to a file, a whole extent is allocated. When writing

to the file again, the data continues where the previous write left off. This reduces

or eliminates file fragmentation.

Since VxFS is an extent-based file system, addressing is done through extents

(which can consist of multiple blocks) rather than in single-block segments. Extents

can therefore enhance file system throughput.

File system disk layouts

The disk layout is the way file system information is stored on disk. On VxFS,

several disk layout versions, numbered 1 through 7, were created to support

various new features and specific UNIX environments. Currently, only the Version

4, 5, 6, and 7 disk layouts can be created and mounted.

Veritas File System features

VxFS includes the following features:

Introducing Veritas File System

Veritas File System features