[Info-vax] Distributed Applications, Hashgraph, Automation

[Stephen Hoffman]

On 2018-02-18 17:59:21 +0000, Jim Johnson said:

> Briefly, what I've seen about the cloud has many aspects, only a few 
> align with outsourcing.  Not sure how much to go into that.
> 
> I was following the discussion on shared-everything vs. shared-nothing 
> structures.  I've used both.  The RMS cache management was pretty 
> expensive to run when I last looked at it.  It was especially bad for 
> high write, high collision rate files.  This drove a different approach 
> with the TP server in DECdtm.  It is structurally a shared nothing 
> service on top of a shared everything access substrate.  It uses an 
> unconventional leader election, in that the 'home' system always is the 
> leader if it is alive, and the other systems elect one of themselves as 
> the leader if it isn't.
> 
> (This was done via a pattern of use with the DLM, and I agree with 
> Steve that either documenting the known patterns or encapsulating them 
> for easier consumption could be useful)
> 
> This produced much better write performance, and good recovery 
> availability times.

That experience is typical.  I've ended up splitting more than a few 
apps similarly.   Either at the volume level, or within the app.   
While SSDs have helped substantially with I/O performance, the 
coordination involved with distributed shared writes ends up limited by 
how fast you can fling lock requests around.    The byte-addressable 
non-volatile storage that's coming on-line right now will only increase 
the coordination load and the likelihood that sharding will be 
considered or required, if you really want to use that memory at speed. 
  Faster networks also cause problems: https://lwn.net/Articles/629155/

> Let me add a caveat on the above.  I've been away from VMS for >15 
> years.  All my data on VMS is very old, and is likely very out of date.

You're still rather current then, with a few errata.  Various of the 
spinlocks have been much better broken up and there've been 
optimizations in the lock management communications implementation and 
elsewhere, and there've been incremental increases to FC HBA speeds and 
NIC speeds, but there've not been significant changes to clustering nor 
to DECdtm and the DLM since the XA-era work, and the cluster 
configuration and management user interface is more or less the same 
though has backslid somewhat in various areas.  Features such as SDN, 
SMB, iSCSI, USB 3.1, UTF-8, and 40 GbE haven't yet arrived in OpenVMS.  
 Some are underway and some are planned: 
https://www.vmssoftware.com/products_roadmap.html


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