VxFS allocates disk space to files in groups of one or more extents. In general, the internal allocation policies of VxFS attempt to achieve two goals: allocate extents for optimum I/O performance and reduce fragmentation. VxFS allocation policies attempt to balance these two goals through large allocations and minimal file system fragmentation by allocating from space available in the file system that best fits the data. These extent-based allocation policies provide an advantage over block-based allocation policies. Extent based policies rarely use indirect blocks with allocations and eliminate many instances of disk access that stem from indirect references.
VxFS allows control over some aspects of the extent allocation policies for a given file by two administrative tools, setext and getext, and an API. The application-imposed policies that are associated with a file are referred to as extent attributes. VxFS provides APIs that allow an application to set or view extent attributes associated with a file and preallocate space for a file.
getext(1) manual pages