[LUDOC-159] Provide more details of "extents" (Glossary)

• Status: Resolved
• Project: Lustre Documentation
• Type: Improvement
• Priority: Minor
• Reporter: Brett Lee (Inactive)
• Assignee: Linda Bebernes (Inactive)
• Resolution: Fixed
• Labels: QContent
• Business Value: 4.26

Description

The term "extents" is used but never defined.

Comments

[Linda Bebernes (Inactive)]

Definition from wikipedia

Extents (ext4)
Extents replace the traditional block mapping scheme used by ext2 and ext3. An extent is a range of contiguous physical blocks, improving large file performance and reducing fragmentation. A single extent in ext4 can map up to 128 MiB of contiguous space with a 4 KiB block size.[1] There can be four extents stored in the inode. When there are more than four extents to a file, the rest of the extents are indexed in an Htree.

In what context are extents relevant to a Lustre user? Would it be sufficient to add more explanatory text to the section 31.2.3 and 31.2.5 and possibly also Sections 32.4.2 and 32.4.3? Other places "extents" is mentioned in the manual aside from code examples are 8.1.1.2 and 36.19.2.
When did Lustre start supporting this feature?

[Linda Bebernes (Inactive)]

Extents are mentioned in these sections in the manual:
8.1.1.2 (installing e2fsprogs)
31.2.3 - table
31.2.5
31.2.10 - table
32.4.2
32.4.3
36.19.2

[Linda Bebernes (Inactive)]

Best solution may be to add a definition of extents to the glossary similar to:
"similar in function to the block mappings in an ext3 inode, an extent is a range of contiguous physical blocks that are mapped via an ext4 inode."

[Linda Bebernes (Inactive)]

A definition of extents has been added to the glossary and is now ready for review at http://review.whamcloud.com/#/c/6990/

[Linda Bebernes (Inactive)]

Changes merged