[gpfsug-discuss] GPFS v5: Blocksizes and subblocks

[Indulis Bernsteins1]

I'm the author of the presentation.

I'll bow to Tomer's knowledge about how the internals of Spectrum Scale 
(GPFS) work.

I've been working with GPFS since V1.3 so it was a bit of a shock to think 
I had a fundamental misunderstanding.

In this case both viewpoints are actually equivalent because of the way 
Spectrum Scale works. Both ways of visualising what happens work in 
exactly the same way from a "user perspective".

The 2 actions of allocating an NSD into a filesystem, and also allocating 
it into a storage pool occur as part of the same single atomic 
transaction. An NSD is either in both a filesystem and a storage pool, or 
it is in neither.

You can visualise one part of the operation first- "allocate NSD into 
filesystem"- and then second part of the operation is"allocate into System 
storage pool within the filesystem" (Stephen's perspective).

Or you can visualise the actions happening the other way around "allocate 
NSD into System storage pool within the cluster", then "allocate into 
filesystem" (Indulis' perspective). The output of mmdf always made me 
think of it in this way.

Because the 2 transactions on the NSD- allocate to filesystem and allocate 
to storage pool- are atomic, and there is a 1:1 mapping in each operation, 
who cares? I can take the viewpoint that the NSD goes into a cluster-wide 
System pool, or someone else can take the view that there is a System pool 
per filesystem. There is no external way to distinguish which is right or 
wrong.

The "visual and mental models" are different but it makes no nevermind in 
terms of how things work.

Though now that I have had to think about it, it is simpler to visualise 
each filesystem having its own System pool, and the fact that Tomer says 
this is how it works internally is a good reason to change the 
visualisation as well :-D

Regards,
 
Indulis Bernsteins
Systems Architect
IBM New Generation Storage

 
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