diff options
| -rw-r--r-- | doc/hacker-guide/en-US/markdown/write-behind.md | 56 | ||||
| -rw-r--r-- | doc/legacy/hacker-guide/write-behind.txt | 45 |
2 files changed, 56 insertions, 45 deletions
diff --git a/doc/hacker-guide/en-US/markdown/write-behind.md b/doc/hacker-guide/en-US/markdown/write-behind.md new file mode 100644 index 00000000000..e20682249e8 --- /dev/null +++ b/doc/hacker-guide/en-US/markdown/write-behind.md @@ -0,0 +1,56 @@ +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. diff --git a/doc/legacy/hacker-guide/write-behind.txt b/doc/legacy/hacker-guide/write-behind.txt deleted file mode 100644 index 50b7d2a1d07..00000000000 --- a/doc/legacy/hacker-guide/write-behind.txt +++ /dev/null @@ -1,45 +0,0 @@ -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 ---------- - - 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. - - |