RE: Laymans explaination for single level store? -- MIDRANGE-L

On Wed, 24 Nov 2004 CWilt@xxxxxxxxxxxx wrote:

First off, David asked for a layman's explanation.  If you want to discuss
technical details, perhaps we should start a new thread.


No, keep this thread going (imo).

But you are correct that in some cases it makes sense that a copy is made so
you can discard changes.  But that's the application level you are talking
about, not the OS level.  Applications on the iSeries can be written to work
with a temporary copy of a given object.  For instance, SEU does this.


This is quite clearly correct.

But that doesn't change the fact that no application can access anything not
in its VM address space.

This is true of all systems and applications (with some exceptions like the raw device linux and other systems have to accomodate oracle). The reason this is true is that the OS (or more precisely the kernel) is that part of the system that is resposible for retrieving the data from all the various devices and getting it to the applications (and vice versa). Applications ask the kernel for memory or disk files or what have you. The kernel does what it can and then makes that data available.

So when dealing with large data files, you have two types of swapping going
on.
1) The application is swapping data from its VM address space to/from the
actual disk file

False. Applications do not swap. The kernel does. Applications do not read the disk. Applications do not allocate swap space. The kernel does all of this. But that is exactly what you are saying is type two below. This type one is non-existent.

2) The OS is swapping VM pages of running applications between main memory
and the swap file on disk.

With single level store, #2 from above is the only swapping happening. You
can see that single-level store has performance benefits.

Type two is the only type that happens regardless of single level store. Single level store has benefits. Performance and sharing may be among them. But not for the reasons outlined above.

James Rich

It's not the software that's free; it's you.
        - billyskank on Groklaw