I doubt this can be called a blog, just a mixture of guesses and statements.
It was a must to have cache mode ON for external pool vols which serve as one of HDT tiers on VSP couple years ago. Recently things were changed and now in case of VSP write pending problems caused by slow external media, you are advised to switch to cache mode OFF for that media, so VSP does not inherit anything from outside of it, stays clean and shining and performs as much as it can as a standalone set of resources. All good. At least from HDS support point of view, such cases are very easy to deal with now - your external storage is slow, go and replace it with something more appropriate to the load you experience.
I am quite a bit in the business and I fully understand that there is no magic anywhere and laws of physics are still in place, so I am fine with this new approach. I believe it is simpler, faster, more reliable and less expensive to say your external storage isn't good enough, so do something with it... Actually sorry, no, we won't leave you with your problems because our Professional Services will be glad to estimate how much the situation is bad and what direction you should take in order to get short-term Holy Grail of Performance. All good again. You have demand we just respond to it.
And so on.. Everything understandable. On the other hand Hitachi is very, very strong organization. It declares principles which have sustained and will sustain existence of this organization for many years ahead because these principles are not just company profit but something which drives people for real life and passion, I mean Pioneer Spirit and other similar "non-money" categories. My hope here is that these Things which are core of Hitachi Heart should prevail over other just money related ones and customer care which has been always one of the best among competitors will reach its glory when
1) people are educated publicly, deeply, widely and timely on system design and performance characteristics and types of system/component behavior along data path;
2) wonderful things Hitachi has come up with, such as HDT, will continue serve different designs and configurations with external storage exploiting their resource as much as possible.
No magic as it was said, "spindle are spindles", however many performance problems and design issues could be eliminated by proper load understanding and clever resource management irrelevantly it is VSP or lower level external storage.
P.S. I know there is some explanation for why HDP pools cannot be used on external HDS arrays for VSP HDT pool vols.. Based on recommendations external pool vols are carved from one RG and even number of these RGs for HDT pool tier is 4, these RGs are not balanced - proved. May be it is time to revise this in parallel with recent cache mode OFF for external HDT pool vols recommendations, so we will get VSP stronger and external storage stronger too simultaneously?