HP OpenVMS Systems

Details of C fflush and of general I/O caching?

» close window

The Question is:

 
Dear Wizard
 
        My code is written in DECC and it'll call fwrite() when it needs.
However, without calling  to fflush() and fsync() , I would like to know
when will  the data
be flushed to disk by VMS.  Thanks.
 
Regards
Wichaya  Charnlertlakha
 
 
 

The Answer is :

 
  DEC C (eventually) passes the data to OpenVMS RMS, and RMS then
  (eventually) writes the data to the disk device, and the disk device
  caches then (eventually) re-orders and writes the data to the physical
  media when each believes it is appropriate to do so.
 
  Writing data out to physical media is very slow, and I/O operations
  are generally reorganized and attempts to avoid or to perform the I/O
  in an efficient manner are made, regardless of the I/O pattern used
  within the application.  Unless OpenVMS and the storage hardware is
  explicitly requested to perform the I/O otherwise, of course.
 
  The default caching behaviours are generally appropriate, but specific
  applications or system configurations may require operations ranging
  from fflush and fsync calls, to RMS sys$flush calls, to the installation
  of (optional) batteries in the StorageWorks shelves to maintain the I/O
  caches over any (transitory) power failures.
 
  Please see previous related discussions here, including "fflush slows
  performance? (rundown)".
 

  

     

     
answer written or last revised on ( 20-APR-1999 )
     
» close window