Re: ensobj

["Larry Loen" <lwloen@xxxxxxxxxx>]

>And while in ups power mode, would force
>all perm object pages to aux
>storage as soon as they were changed.

The question here is not entirely "is every change moved out to aux
storage"?

The question is "were the changes coherent according to the OS or (for
persistent user spaces) the application that was manipulating them?"

Coherence in a persistent object system is the AND of "moved everything out
to disk" and "code got to a logical boundary-point such that all pointers
were set, all numbers added up, etc."

To be sure, moving everything out to disk can help, but it isn't enough in
itself and, in some cases, may make no practical difference.  Suppose the
machine crashes in the middle of updating a complex tree.  Suppose the tree
is implemented in a persistent *USRSPC created by application code.
Suppose further that the code has no idependent "journal" type scheme to
record each change as it happens such that it can be undone after some sort
of crash (job or system).  Now, exactly how is the object "safe" and
"useful" even if it is pushed out to disk "no matter what" under these
circumstances?


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


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