The Question is:
What is a good value of the 'Map area words in use' from a DUMP/HEADER command
for an INDEXF.SYS file to watch for to trigger an offline defrag process? Is
there a maximum value?
The Answer is :
The maximum size of the map area is 150 words. Each mapping pointer
takes up 2 or 3 words. The Wizard therefore recommends anything over
140 words to be the "red zone" for INDEXF.SYS, indicating that an
offline defrag should be performed (either using BACKUP/IMAGE and restore,
or using the DEFRAGMENT OFFLINE_VOLUME command in the Disk File Optimizer
Note that wizard recommends *against* preallocating header space to
INDEXF.SYS as it is very difficult to recover overallocations. If you
have a disk with a full INDEXF.SYS, you will probably only need *ONE*
BACKUP/IMAGE and restore over the life of the disk to enable INDEXF.SYS
to expand to a size where it is possible to fill the disk. The Wizard
has never come across a disk which has filled INDEXF.SYS twice.
The reason for this problem in the first place was the algorithm for
extending INDEXF.SYS. In OpenVMS V5.5-2 and below, it was exceedingly
simple - a constant 1000 blocks. Moreover, there was no particular effort
to allocate a contiguous extent. This algorithm was fine for disk sizes
common at the time it was implemented, but has proven inadequate for
modern, high capacity disks (say over 1GB).
In OpenVMS V6 and higher, the extension algorithm takes into account the
average file size on the disk and the amount of free space. It calculates
the estimated size of INDEXF.SYS required to fill the disk and extends
to that size. Furthermore, the extension is contiguous-best-try. The
wizard is yet to see a single cases of INDEXF.SYS filling up on disks
initialized on OpenVMS V6 or higher.