HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

24 Chapter1

The VxFS File System

Extent-Based Allocation

inodes also use an 8K default indirect data extent size but allocate the

indirect data extents in clusters to simulate larger extents.

Typed Extents

NOTE The information in this section applies to the VxFS Version 3 and 4 disk

layout.

In Version 3 and 4, VxFS introduced a new inode block map organization

for indirect extents known as

typed extents

. Each entry in the block

map has a typed descriptor record containing a type, offset, starting

block, and number of blocks.

Indirect and data extents use this format to identify logical ﬁle offsets

and physical disk locations of any given extent. The extent descriptor

ﬁelds are deﬁned as follows:

• Indirect address blocks are fully typed and may have variable lengths

up to a maximum and optimum size of 8K. On a fragmented ﬁle

system, indirect extents may be smaller than 8K depending on space

availability. VxFS always tries to obtain 8K indirect extents but

resorts to smaller indirects if necessary.

• Indirect Data extents are variable in size to allow ﬁles to allocate

large, contiguous extents and take full advantage of VxFS's optimized

I/O.

• Holes in sparse ﬁles require no storage and are eliminated by typed

records. A hole is determined by adding the offset and length of a

descriptor and comparing the result with the offset of the next record.

• While there are no limits on the levels of indirection, lower levels are

expected in this format since data extents have variable lengths.

• This format uses a type indicator that determines its record format

and content and accommodates new requirements and functionality

type Uniquely identiﬁes an extent descriptor record and

deﬁnes the record's length and format.

offset Represents the logical ﬁle offset in blocks for a given

descriptor. Used to optimize lookups and eliminate

hole descriptor entries.

starting block The starting ﬁle system block of the extent.

number of blocks The number of contiguous blocks in the extent.