[Torsion-dev] Storage -- segments & pages

[Davison Avery]

Dan, 

Great work on TorsionOS. I was reading your wiki page about the
page-based storage system. 

I have a few preliminary questions, without having yet gone through your
source (which I plan to do):

1. When you are checkpointing, I want to confirm that you only commit
the dirty pages that correspond to a specific point in time, since a
checkpoint (if I am not mistaken) should correspond to the state of
virtual memory at a specific instant in time. 

2. With regards to your disk layout, why did you choose to use segments
and pages? Was it because pages seem to be more low-level and are more
dependent on block size, while segments were meant to be more abstract?
I don't see the reason why the disk layout couldn't have stayed at the
page level. 

EDIT: After reading your paragraph on Checkpointing, it seems to me that
you are treating segments as a way to track disk utilization only; while
pages are specifically virtual address space related only. Based on this
understanding, would it be correct to indicate that segments would (for
a 32bit system) be 4GB in size (not including supeblock sizes), and that
at minimum the whole system would require two segments -- one for
checkpointing all dirty pages, and another segment to hold the last
successful checkpointed image? 



Thanks!