[gpfsug-discuss] GPFS v5: Blocksizes and subblocks
Indulis Bernsteins1
INDULISB at uk.ibm.com
Wed Mar 27 18:31:24 GMT 2019
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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