[Info-vax] New filesystem mentioned

[Chris Scheers]

Dave Froble wrote:
> On 5/14/2019 4:52 PM, Chris Scheers wrote:
>> Bob Koehler wrote:
>>> In article <qbco24$r22$2 at dont-email.me>, Dave Froble
>>> <davef at tsoft-inc.com> writes:
>>>> Not that I'm an expert on VMS clusters, but it has been my impression
>>>> that the OS did all the work.  But what do I know?
>>>
>>>    The file system is very much part of the OS.  If it's going to work
>>>    in VMSclusters, it must support VMSclusters.
>>
>> There seems to be some heavily blinkered, all-or-nothing thought going
>> on in this discussion.
>>
>> There were file systems in VMS before clusters and those file systems
>> still work.
>>
>> For example, it should be possible to side step the DLM by doing a
>> local, ACP based file system and MSCP serving the results to the cluster.
> 
> This really isn't about the DLM.  It's more about coordination.  The DLM 
> is a tool that can be useful for that.
> 
> Even a file system running on one node still needs some type of locking.
> 
>> Obviously, this looses the advantages of being cluster aware, but it
>> does provide the advantages of the file system without needing to go
>> down the DLM rabbit hole.
> 
> What would such a filesystem be used for, without some form of locking 
> available?
> 
>> So the real question is: What are the advantages of being cluster aware
>> vs. the advantages of the file system?
>>
>> Having both is best, but is there a viable compromise that is useful?
>>
>> I think that being useful, even with limitations, is a whole lot better
>> than having nothing.
> 
> 
> The real rabbit hole is caching.  Let me tell you, if you're using 
> caching, you will not give it up.  Even on a single system the 
> difference with caching on or off is like night and day.
> 
> And the coordination of not just what's "on disk" but also what's in the 
> cache(s) can be rather non-trival.  It scared us when we looked at it. 
> It's even complex on a single system.

That's the point of an ACP.  (And the problem with an ACP.)  All I/O for 
all users goes through a single process.  There is no need for the DLM.

The ACP is responsible for its caching.  The ACP is a weird mixture of 
process context and device driver context.  It has access to the various 
synchronization mechanisms available to both processes and drivers.  It 
can use the DLM, but it does not have to.

-- 
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Chris Scheers, Applied Synergy, Inc.

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