VMS/IRAF Installation and Site Manager's Guide

VMS/IRAF Installation and Site Manager's Guide

8.3. Rendering large runtime files contiguous

The speed with which large disk files can be read into memory in VMS can degrade significantly if the disk is highly fragmented, which causes a large file to be physically stored in many small fragments on the disk. IRAF performance as well as VMS system throughput can therefore be improved by rendering frequently referenced large files contiguous. Examples of files which it might pay to render contiguous are the executables in bin, all of the runtime files in dev, and the large system libraries in lib (this last is only useful to speed software development, i.e., linking).

The easiest way to render a file contiguous in VMS is to use COPY/CONTIGUOUS to copy the file onto itself, and then purge the old version. Various nonstandard utility programs are available commercially which will perform this function automatically. The contents of an entire disk may be rendered contiguous or nearly contiguous by backing up the disk onto tape and then restoring it onto a clean disk.