VERITAS File SystemÖ 3.5 (HP OnlineJFS/JFS3.5) AdministratorÆs Guide (December 2002)

Chapter 1

The VERITAS File System

Extent Based Allocation

Disk space is allocated in 1024-byte sectors to form logical blocks. VxFS supports logical block sizes of 1024,

2048, 4096, and 8192 bytes. The default block size is 1K.

An extent is defined as one or more adjacent blocks of data within the file system. An extent is presented as an

address-length pair, which identifies the starting block address and the length of the extent (in file system or

logical blocks). VxFS allocates storage in groups of extents rather than a block at a time.

Extents allow disk I/O to take place in units of multiple blocks if storage is allocated in consecutive blocks. For

sequential I/O, multiple block operations are considerably faster than block-at-a-time operations; almost all

disk drives accept I/O operations of multiple blocks.

Extent allocation only slightly alters the interpretation of addressed blocks from the inode structure

compared to block based inodes. A VxFS inode references 10 direct extents, each of which are pairs of starting

block addresses and lengths in blocks. The VxFS inode also points to two indirect address extents, which

contain the addresses of other extents:

• The first indirect address extent is used for single indirection; each entry in the extent indicates the

starting block number of an indirect data extent.

• The second indirect address extent is used for double indirection; each entry in the extent indicates the

starting block number of a single indirect address extent.

Each indirect address extent is 8K long and contains 2048 entries. All indirect data extents for a file must be

the same size; this size is set when the first indirect data extent is allocated and stored in the inode. Directory

inodes always use an 8K indirect data extent size. By default, regular file inodes also use an 8K indirect data

extent size that can be altered with vxtunefs (see “Tuning VxFS I/O Parameters” on page 29); these inodes

allocate the indirect data extents in clusters to simulate larger extents.