Re: [RFC] mount flag "direct" (fwd)

[Joachim Breuer <jmbreuer@gmx.net>]

Helge Hafting <helgehaf@aitel.hist.no> writes:

> Joachim Breuer wrote:
>> > Well, one has to read it from scratch. I'll set about seeing how to do.
>> > CLues welcome.
>> 
>> Just an idea, I don't know how well this works what with the 'IDE
>> can't do write barriers right' and related effects:
>> 
>> - Allow all nodes to cache as many blocks as they wish
>> - The atomic operation "update this block" includes "invalidate this
>>   block, if cached" broadcast to all nodes
>> 
> You can't just invalidate like that, you'll need synchronization.
> Something like "I want write access to this block - stop using it."
> And then you _wait_, until everybody else has released it.  This
> could take some time if someone was busy using it.

More or less what I meant - I assumed the requirement of mutual
exclusion was already agreed upon. I might have phrased it more
clearly as in "the locking protocol provides the "other" nodes with
sensible invalidation data". Something which a "budgeting" type
locking protocol would not normally do (allow each node a range of its
own for exclusive writes; everyone can read everywhere (with a bit
more locking to make the writes look atomic if they aren't already
perhaps)).

>> Performance would certainly become an issue; depending on the
>> architecture bus sniffing as in certain MP cache consistency protocols
>> might be feasible (I, node 3, see a transfer from node 1 going to
>> block #42, which is in my cache; so I update my cache using the data
>> part of the block transfer as it comes by on the bus).
>
> Possible only when there is a shared bus.  Of course today's
> IDE/SCSI don't do this.

Well, SCSI can be *made* to do it (actually, this is what I had in
mind when I wrote it up) - but I don't know whether any mainstream
controllers would be able to "sniff" sensibly.


Anyway, so long,
        Joe

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