low power actions?

[Edward Cherlin]

What would it take to put in a journaling filesystem?

On Fri, Jul 11, 2008 at 10:49 AM, Eben Eliason <eben.eliason at gmail.com> wrote:
> On Fri, Jul 11, 2008 at 1:33 PM, Gary C Martin <gary at garycmartin.com> wrote:
>> One concern I have with auto saving state before powering off is the
>> potential corruption of journal data. How robust is the Journal if
>> power off happens half way through an ongoing auto state save – do you
>> loose both the new journal entry and the original entry you had
>> resumed from (partially overwritten)?
>
> Disclaimer:  I'm not a technical expert on the DS, so others more
> familiar should probably correct me if I make claims below that are
> false.
>
> This is yet another problem that can be bypassed with the "new DS".
> In one of our past meetings, we laid out requirements for the process
> by which activities save their state, and it included a means for
> activities to check in temporary saves if they wished to, optionally
> passing a flag to tell the Journal to actually create a new entry.
> This system was in place such that, if the Journal detected that a
> given activity crashed, it could automatically make a new Journal
> entry based on the last temporary save, as a form of auto-recovery.
> This approach could similarly be used after a power failure.
>
> Additionally, in the worst case a corrupt entry might wind up in the
> Journal, but that shouldn't be a problem because, at present, copies
> are stored so there is no loss of data, and in the future we'll have
> versions, and only one version of many would be corrupt.  It should
> never be the case that the entry that was opened gets corrupted.
> Ideally the Journal would be able to recognize when a save transaction
> doesn't finish and either replace it with the most recent temporary
> state or remove the entry compeletely.
>
> - Eben
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-- 
Edward Cherlin
End Poverty at a Profit by teaching children business
http://www.EarthTreasury.org/
"The best way to predict the future is to invent it."--Alan Kay