[Info-vax] What is a section file? How is it helpful? Is a TPU$SECTION file a section file?

[Stephen Hoffman]

On 2022-08-31 00:07:29 +0000, Arne Vajhj said:

> On 8/30/2022 6:10 PM, Stephen Hoffman wrote:
>> 
>> A section file...
>> 
>> ...is a form of persistent storage available for in-memory data structures.
>> 
>> ...can be used to persist a user-written or third-party in-memory 
>> database.  Or can persist a pile of shared data structures. Or other 
>> such.
>> 
>> ...lacks any database-like APIs and features; it's a means to map 
>> (transfer) arbitrary user data from persistent storage into memory, and 
>> from memory into persistent storage.
>> 
>> ...sections are the underpinning of Fortran COMMON, the C/Unix mmap() 
>> APIs, of parts of TPU customizations, and related mechanisms.
> 
> I agree with everything except the Fortran common block thing.
> 
> A Fortran common block is a psect, but not this kind of section (per 
> default - it can of course be mapped like any other memory).

Sure.

It's a section of virtual memory with backing (and sharable) storage, 
with the salient difference being whether it uses static 
mapping—Fortran, BASIC, C, etc., common storage involving the linker—or 
use dynamic mapping—run-time calls to $crmpsc, etc.

PFN mapping also differs in details, too.

The whole thing is lacking an abstraction layer or two, but entirely 
workable for those building their own tooling.

Persisting data with sections is Less Than Fun; a local or distributed 
transaction log—RAIS—may or will become interesting, even with local 
persistent storage, as flushing pages of virtual memory to persistent 
storage can get tricky, and that's before we get to outage-related 
details such as 8 KiB storage mapped to half KiB storage sectors, etc. 
Persistent memory can't get here fast enough for these and related 
tasks, not that Intel didn't just bail out of that whole Optane market, 
seemingly now betting on storage access via CXL.



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