[Info-vax] Databases versus RMS

[Bob Koehler]

In article <jmmds1$cfl$1 at Iltempo.Update.UU.SE>, Johnny Billquist <bqt at softjar.se> writes:
> 
> Hmm. If that is so, then VMS works rather different from RSX. In RSX, 
> disk caching in the OS happens under the QIO layer. Do VMS not have disk 
> caching? Or how does it deal with sharing between different processes, 
> where I/O is hitting the same actual disk blocks?
> 
> Or was that what you meant by the writeback caching comment?

   Sharing is generally done through RMS layers.  RMS will cache as
   much or as little as it is told to.  Below RMS, VMS is using
   write-through cache.  HLL RTLs can also cache, but I don't know
   if any do using other than RMS features.

   I never experienced anything but write-through cache on RSX, but I
   never did know it as well.

   Possibly _the_ exception is the pre-allocated block cache.  VMS
   will mark blocks used on the drive that have not been used in order
   to keep a cache of free blocks for upcoming writes.  This is what
   gets rebuilt when an improperly dismounted disk is mounted without
   /norebuild, or when a "set volume/rebuild", is done.