Veritas™ File System 5.0.1 Administrator's Guide

Used for single indirection. Each entry in the extent indicates the
starting block number of an indirect data extent
first
Used for double indirection. Each entry in the extent indicates the
starting block number of a single indirect address extent.
second
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; these inodes allocate the
indirect data extents in clusters to simulate larger extents.
Typed extents
VxFS has an 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:
Identifies uniquely an extent descriptor record and defines the record's
length and format.
type
Represents the logical file offset in blocks for a given descriptor. Used
to optimize lookups and eliminate hole descriptor entries.
offset
Is the starting file system block of the extent.starting block
Is the number of contiguous blocks in the extent.number of blocks
Typed extents have the following characteristics:
■ 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 optimized I/O in VxFS.
■ 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.
19Introducing Veritas File System
Veritas File System features