VERITAS File System 4.1 Administrator's Guide

The VERITAS File System
Extent Based Allocation
Chapter 118
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 IK.
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. 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 57 ). These
inodes allocate the indirect data extents in clusters to simulate larger extents.