Messages in this thread |  | | Subject | Re: [RFC] mount flag "direct" (fwd) | From | Joachim Breuer <> | Date | Wed, 04 Sep 2002 23:26:08 +0200 |
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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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