Veritas File System 5.1 SP1 Administrator"s Guide (5900-1499, April 2011)

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.

■ 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 for future types.

The current typed format is used on regular files and directories 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.

Extent attributes

VxFS allocates disk space to files in groups of one or more extents. VxFS also

allows applications to control some aspects of the extent allocation. Extent

attributes are the extent allocation policies associated with a file.

The setext and getext commands allow the administrator to set or view extent

attributes associated with a file, as well as to preallocate space for a file.

See the setext(1M) and getext(1M) manual pages.

Introducing Veritas File System

Veritas File System features