[Info-vax] OS implementation languages

[Bob Gezelter]

On Thursday, August 31, 2023 at 4:58:13 PM UTC-4, Robert A. Brooks wrote:
> On 8/31/2023 3:32 PM, Dave Froble wrote: 
> 
> > Making your database files contiguous seems to be a winner.  Larger clustersize 
> > for database files also helps, a lot.
> Cluster size is not used for I/O; it's only used for storage allocation, where 
> a single bit in the storage allocation bitmap covers the number of blocks 
> in that volume's clustersize. 
> 
> I/O is not done in clustersize chunks. 
> 
> -- 
> 
> --- Rob
Rob,

WADR to Dave, he omitted some steps. When Hein was giving DECUS and Bootcamp presentations on optimizing files, he took note of the fact that newer storage arrays worked with 16 block chunks internally. If I recall correctly, he recommended matching the cluster factor and other I/O-related parameters, e.g., bucket size and buffer sizes to the same granularity. 

While it does not guarantee a lack of split I/Os it dramatically reduces their frequency, much as aligning quadwords and longwords improves program performance.

I have often encountered logical volumes that originated on RP and RM-class disk drives originally on VAX systems, with cluster sizes set to track and integral divisors of tracks. In the same way,  they reduce splits. Of course, when these volumes are migrated to present technologies, the parameters require reconsideration and adjustment for optimal performance.

You are correct that there is no direct connection between cluster size and I/O request size. However, the lack of a direct connection does not prevent one from arranging it to be the most frequent case.

- Bob Gezelter, http://www.rlgsc.com