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Date: Fri, 25 Jul 2008 20:40:34 +0100 From: Jamie Lokier <jamie@...reable.org> To: Evgeniy Polyakov <johnpol@....mipt.ru> Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org Subject: Re: [0/3] POHMELFS high performance network filesystem. IPv6 support, documentation update. >From the design notes, > POHMELFS got full data and metadata cache coherency support. > > It was rather simple task due to async event processing support. > > Each time client creates, reads or writes object to server, > information about its interest is stored on server. When any other > client updates the same object (like changing attributes or writes > data), all interested clients get notifications with new data (new > attributes, or in case of writing possibly new size and flag, which > page has to be fetched from the server, since it is not valid > anymore). Writing happens during writeback as before, so commands like > "echo Some_message > /mnt/file" immediately syncs size of the file to > zero and after some time writes there actual data, when system will > decide to start writeback. I'm just going by what the notes say, which don't seem very clear. Consider this: 1. Client A reads FILE, and registers its interest in FILE. (Contents are not interesting, e.g. 'Hello_sister') 2. Client B does "echo Some_message > /mnt/file". - Truncates the file, sending truncate message to server. - "Writing happes during writeback"...? 3. Client B sends a message by back-channel to client A (e.g. ssh command). 4. Client A reads FILE again. Does client A always see 'Some_message' when it reads the file in step 4? That's what I'd call coherence. For that, the first truncate or write operation on client B must wait until a synchronous invalidate request goes to the server, then the server sends to all interested clients (A) and waits for a reply, then reply to B, and only then can B return from the open()/write() system call. And when client A reads the file in step 4, it must send a synchronous message to the server which must ask B to write the delayed writeback data immediately, and until then, the reply to A will be delayed. Is that right? Thanks, -- Jamie -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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