18.8.1 Disk Defragmentation

When a disk is first initialized, its available free space is contained in one large contiguous area. When files are created, they are allocated from this contiguous pool and, thus, are also contiguous. As more files are created, deleted, and extended, available free space becomes more fragmented. Eventually, files must be allocated to space made up of multiple fragments located in different physical regions of a disk. When users perform an I/O operation to a fragmented file, OpenVMS may be forced to break a single operation into two or more operations a split I/O to read the multiple fragments.

A file is made up of one or more extents. Each extent describes each fragment of a file. A contiguous file has only a single extent, whereas a severely fragmented file might have several hundred extents. OpenVMS can access only a limited number of extents within a file at one time. By default, this limit is defined by the system parameter ACP_WINDOW, which has a default value of 7. If a file has more than seven fragments, and a user attempts to access a fragment not described by the current window, OpenVMS must perform a window turn and retrieve information about the unmapped fragment. An excessive window-turn rate is an indication of a severe fragmentation problem.

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