Symantec logo

Extent attributes

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 via two administrative tools, setext(1) and getext(1), and an API. The application-imposed policies 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.

Attribute specifics

There are two basic extent attributes associated with a file: reservation and fixed extent size. You can preallocate space to the file by manipulating a file's reservation, or overriding the default allocation policy of the file system by setting a fixed extent size. Other policies determine the way these attributes are expressed during the allocation process. You can specify that:

Some of the extent attributes are persistent and become part of the on-disk information about the file, while other attributes are temporary and are lost after the file is closed or the system is rebooted. The persistent attributes are similar to the file's permissions and are written in the inode for the file. When a file is copied, moved, or archived, only the persistent attributes of the source file are preserved in the new file.