doc: Convert the write-behind translator documentation to markdown

Change-Id: I213e230d283cd43bd1c2b2948882ab36f9170608
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5482
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 56 insertions, 0 deletions

diff --git a/doc/hacker-guide/en-US/markdown/write-behind.md b/doc/hacker-guide/en-US/markdown/write-behind.md
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+performance/write-behind translator
+===================================
+
+Basic working
+--------------
+
+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
+---------
+
+With 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.