>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
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