Understanding Fragmentation

When free resources are initially allocated to files in a VERITAS file system, they are aligned in the most efficient order possible to provide optimal performance. On an active file system, the original order is lost over time as files are created, removed, or resized. As space is allocated and deallocated from files, the available free space becomes broken into fragments. This means that space must be assigned to files in smaller and smaller extents. This process is known as fragmentation. Fragmentation leads to degraded performance and availability. The degree of fragmentation depends on file system usage and activity.

Controlling Fragmentation

VxFS provides online reporting and optimization utilities to enable you to monitor and defragment a mounted file system. These utilities are accessible through the file system administration command, fsadm. Using the fsadm command, you can track and eliminate fragmentation without interrupting user access to the file system.

Types of Fragmentation

VxFS addresses two types of fragmentation:

• Directory Fragmentation

As files are created and removed, gaps are left in directory inodes. This is known as directory fragmentation. Directory fragmentation causes directory lookups to become slower.

• Extent Fragmentation

As files are created and removed, the free extent map for an allocation unit changes from having one large free area to having many smaller free areas. Extent fragmentation occurs when files cannot be allocated in contiguous chunks and more extents must be referenced to access a file. In a case of extreme fragmentation, a file system may have free space that cannot be allocated.