VERITAS File System 4.1 Administrator's Guide

The VERITAS File System
Extent Based Allocation
Chapter 1 19
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:
• 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 since data extents have variable
lengths lower levels are expected in this format.
• This format uses a type indicator that determines its record format and content and
accommodates new requirements and functionality for future types.
The current typed format is used on regular files only when indirection is needed. Typed
records are longer than the previous format and require less direct entries in the inode. Newly
created files start out using the old format which allows for ten direct extents in the inode.
The inode's block map is converted to the typed format when indirection is needed to offer the
advantages of both formats.
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.