HP OpenVMS Systems

Buffered vs Direct I/O?

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The Question is:

 
Without global buffers, how does VMS decide how much buffered and direct io its
 going to perform on a file that is accessed sequentially? Does fragmentation,
 extent sizes, directory sizes influence this decision? Is the decision made
 once only when the fi
le is opened? What would make VMS decide to do no buffered io?
 

The Answer is :

 
  Buffered I/O operations (BUFIO, in the context on RMS indexed file
  access) typically indicate file-level operations, notably EXTENTs.
  Excessive numbers imply an inadequately-tuned file.
 
  Global buffers do not particularly effect this, as I/O operations
  involving an RMS global buffer are direct I/O operations -- directly
  analogous to the I/Os used for local buffer operations.
 
  RMS global buffers are very efficient for both read- and write-accessed
  files, whether clustered or not.  Enabling and using global buffers
  tends to be helpful in most cases, and are often of significant benefit
  on files that are heavily accessed.
 
  Please recognize that in order to $PUT a record to a file, RMS has to
  READ through several layers of index structure and had to read the
  target bucket.  All of this could be found in a gloabl buffer, when
  the file is correctly configured.  (Hint: Global buffers are often
  specifically sized to hold the entire index structures of a file.)
 
  If a buffer to be written is found in the global buffer, it is simply
  used there and written out from there.  If the buffer was not available,
  only then might RMS choose to read it into a local buffer, but that is
  exactly what would have happened had there not been global buffers.  No
  particular loss, only a potential performance gain.
 
  Only deferred write will muddle the picture some, as this will force
  RMS to use a local buffer. If the target is found in the global buffer,
  it will do a memory copy to local and will discard the global buffer
  original, again no loss.
 
  The only real price for global buffers tends to be the global buffer
  lock, which can increase the lock enqueue and dequeue activity -- with
  recent versions of OpenVMS and RMS, the aggregate performance of this
  serialization logical has been vastly improved.
 

  

     

     
answer written or last revised on ( 2-APR-2003 )
     
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