1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
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.
|
