Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

The VERITAS File System
Extent Based Allocation
Chapter 1 7
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.