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ODS-2/ODS-5 disk (storage bitmap) internals?

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The Question is:

I am trying to write a program to read the BITMAP.SYS file on an ODS-2 disk.  I
 have come to realise that it is returning incomplete information.  I suspect
 that this is due to some of the live data being in a cache rather than in the
 file.  How do I get
the cache flushed back to the file?  Or is there a system service I should be
 using to access the BITMAP?  Can you give me an example, please?  I am writing
 the program in COBOL but an example in (almost) any language would do since, I
 suspect, it's going
 to require mainly calls to System Services.  Thanks

The Answer is :

  Please acquire the available VMS File System Internals book for details
  on the internals and on-disk structures of the ODS-2 file system.
  Please be aware that ODS-2 and ODS-5 disks were changed in OpenVMS V7.2
  to optionally permit larger bitmaps.  This change permits correspondingly
  smaller disk cluster factors as disk volume sizes increase; disk cluster
  factors rather smaller than were permitted on earlier OpenVMS releases.
  Available tools such as ANALYZE and DFU can be used to access the storage
  bitmap -- the bitmap itself is simply a section of disk blocks, and it is
  directly accessable via sys$qio[w] logical block access.  While there are
  caches used for the storage bitmap, the disk quota file, and various other
  constructs, OpenVMS does strive to keep the contents of these various disk
  caches consistent with the contents of the disk itself.
  Without specific details on the "incomplete information" being reported,
  a more direct or specific answer is difficult.

answer written or last revised on ( 28-JUL-2000 )

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