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Veritas File System (VxFS) allocates disk space to files in groups of one or more adjacent blocks called extents. VxFS defines an application interface that allows programs to control various aspects of the extent allocation for a given file. The extent allocation policies associated with a file are referred to as extent attributes.
The VxFS getext and setext commands let you view or manipulate file extent attributes.
Because these facilities are provided using VxFS-specific IOCTL system calls, most existing UNIX system applications do not use them. The cp, cpio, and mv utilities use the facilities to preserve extent attributes and allocate space more efficiently. The current attributes of a file can be listed using the getext command. The facilities can also improve performance for custom applications. For portability reasons, these applications must check which file system type they are using before using these interfaces.
See the setext(1) and getext(1) manual pages.
The two basic extent attributes associated with a file are its reservation and its fixed extent size. You can preallocate space to the file by manipulating a file's reservation, or override the default allocation policy of the file system by setting a fixed extent size.
See Reservation: preallocating space to a file.
See Fixed extent size.
Other policies determine the way these attributes are expressed during the allocation process.
You can specify the following criteria:
The space reserved for a file must be contiguous
No allocations will be made for a file beyond the current reservation
An unused reservation will be released when the file is closed
Space will be allocated, but no reservation will be assigned
The file size will be changed to incorporate the allocated space immediately
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.
See Other extent attribute controls.
In general, the user will only set extent attributes for reservation. Many of the attributes are designed for applications that are tuned to a particular pattern of I/O or disk alignment.