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basic working
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write behind is basically a translator to lie to the application that the write-requests are finished, even before it is actually finished.
on a regular translator tree without write-behind, control flow is like this:
1. application makes a write() system call.
2. VFS ==> FUSE ==> /dev/fuse.
3. fuse-bridge initiates a glusterfs writev() call.
4. writev() is STACK_WIND()ed upto client-protocol or storage translator.
5. client-protocol, on receiving reply from server, starts STACK_UNWIND() towards the fuse-bridge.
on a translator tree with write-behind, control flow is like this:
1. application makes a write() system call.
2. VFS ==> FUSE ==> /dev/fuse.
3. fuse-bridge initiates a glusterfs writev() call.
4. writev() is STACK_WIND()ed upto write-behind translator.
5. write-behind adds the write buffer to its internal queue and does a STACK_UNWIND() towards the fuse-bridge.
write call is completed in application's percepective. after STACK_UNWIND()ing towards the fuse-bridge, write-behind initiates a fresh writev() call to its child translator, whose replies will be consumed by write-behind itself. write-behind _doesn't_ cache the write buffer, unless 'option flush-behind on' is specified in volume specification file.
windowing
---------
write respect to write-behind, each write-buffer has three flags: 'stack_wound', 'write_behind' and 'got_reply'.
stack_wound: if set, indicates that write-behind has initiated STACK_WIND() towards child translator.
write_behind: if set, indicates that write-behind has done STACK_UNWIND() towards fuse-bridge.
got_reply: if set, indicates that write-behind has received reply from child translator for a writev() STACK_WIND(). a request will be destroyed by write-behind only if this flag is set.
currently pending write requests = aggregate size of requests with write_behind = 1 and got_reply = 0.
window size limits the aggregate size of currently pending write requests. once the pending requests' size has reached the window size, write-behind blocks writev() calls from fuse-bridge.
blocking is only from application's perspective. write-behind does STACK_WIND() to child translator straight-away, but hold behind the STACK_UNWIND() towards fuse-bridge. STACK_UNWIND() is done only once write-behind gets enough replies to accomodate for currently blocked request.
flush behind
------------
if 'option flush-behind on' is specified in volume specification file, then write-behind sends aggregate write requests to child translator, instead of regular per request STACK_WIND()s.
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