Re: [concept & "good taste" review] persistent store

[Tony Luck]

On Sat, Dec 18, 2010 at 10:23 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> You want to have a ring of events, and into that ring you also have a
> "this event has been read" pointer. And you _never_ overwrite entries
> that haven't been read yet, because quite frankly, if you get some
> nasty memory corruption, you may end up with a thousand oopses in
> rapid succession, and the latter ones are likely to be just fallout
> from the earlier ones. So you definitely don't want to overwrite the
> earlier ones, because they are more likely to contain the clues about
> the actual original cause.
>
> At the same time, you do want to have the capability of saying "I've
> seen this", and let it be overwritten. For example, if we end up
> teaching syslogd or something like that to use this, syslogd would
> write the oops to disk, do a fdatasync() on the oops file, and after
> it's stable on disk it can mark it "read".
>
> Also, since this is very much about persistent storage, I think any
> events from a previous boot that still exists should be marked "read".
> You still want to be able to read them (so marking something "read"
> does not mean that it goes away), but if a new oops happens, you don't
> want some old entries from long ago to stop it from being written to
> persistent storage. So if you don't have any syslogd or any other tool
> that saves things to disk, you'd still get the new oopses into
> persistent storage.
>
> Doesn't that sound like the best of both worlds?

It sounds like an excellent heuristic for how the platform layer
should manage the persistent store when space is tight. But
I think that I can still keep my /dev/pstore filesystem as a
presentation layer to make the bits available to the user in
a device independent way.

Or perhaps the pstore layer can help with the implementation
of the heuristic. It knows what items are in the pstore, so it
could build & maintain the "ring" and pass the id of the least
wanted item down to the platform layer whenever it wants to
write a record ... with the platform layer giving a status to
say whether it had to delete that item to make space for the
new one?

-Tony
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