Re: teraspace, user spaces, etc.

The size of allocation and the size you ask for are not necessarily
related.  You get the virtual store, but that says nothing about the disk.

If you ask for that sort of storage, the usual practice is to try and find
big extents.

However, this is something we've tinkered with a lot over the years and I
can't say exactly how it works today.  We have always, however, broken up
things that big into littler actual allocations when necessary.  In other
words, we are under no obligation and never have required ourselves to
allocate objects contiguously, nor even on one physical volume.  That's why
there are ASPs; to allow nearly everything to be on mulitple volumes (with
some ability for logical aggregation not tied to physical boundaries).
This practice goes all the way back to the beginning of S/38.

I would expect, therefore, that something as big as 1 MB may or may not be
contiguous.  All those kind of ties to contiguous space and physical
volumes I always thought was slow, artificial, and confining
performance-wise.  Single Level Store gave us an excuse to get rid of all
that.  Really big, contiguous allocations are not as beneficial as one
might think, performance-wise.

Now, obviously, if we aren't constrained on the initial allocation, neither
are we when we extend (make something bigger).  We just add new space
allocated under very similar rules to creating it originally.


Larry W. Loen  -   Senior Java and iSeries Performance Analyst
                          Dept HP4, Rochester MN


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