HP-UX HB v13.00 Ch-14 - JFS

HP-UX Handbook – Rev 13.00 Page 7 (of 47)

Chapter 14 Journaled File System (JFS)

October 29, 2013

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 blocks 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.

The on-disk layout of VxFS is versioned and upgradeable while the file system is mounted. This

file system has gone through seven versions.

 Version 2 added support for filesets, dynamic inode allocation and ACLs. Layouts 1-3

stopped being supported in VxFS 4.0.

 Version 4 added support for storage checkpoints and for Veritas Cluster File System.

Version 4 was released in VxFS 3.2.1. Layout version 4 is no longer supported under

VxFS 5.1

 Version 5 started support for file systems up to 32 terabytes (2

bytes) in size.

 Individual files can be up to 2 terabytes in size. Version 5 was introduced in VxFS 3.5

and is no longer supported under VxFS 5.1

 Version 6 added support for file systems and files up to 8 exabytes (2

bytes) in size.

Version 6 also introduced support for named streams/resource forks, for multiple

underlying volumes, and for file change logs. Version 6 was introduced in VxFS 4.0.