Re: Power HA and Journaling.

On 1/29/2015 2:52 PM, Horn, Jim wrote:

I understand the performance penalty, but
I guess somehow bundling journal writes (not making them when they happen)
is different from not writing data changes when they happen?
Still baffles me. I guess IBM just keeps it straight. The more it's like
magic........

If I may...

With FRCRATIO(1), a database write looks like this:
INSERT INTO...
call database I/O API
wait for disk to acknowledge the write
return to INSERT

With journaling, the same transaction looks like this:
INSERT INTO...
call database I/O API
don't wait
call journal API, which runs in different process
don't wait
return to INSERT

So, while journaling seems to do an extra thing, it does it in parallel
with the rest of the I/O. I think the key word here is asynchronous.