proxy_buffering off causes truncated responses when backend emits response in small chunks

[Maxim Dounin]

Hello!

On Tue, Feb 07, 2012 at 03:54:08PM -0800, W. Andrew Loe III wrote:

> We use nginx as both a load-balancer and webserver. This issue is with the
> nginx functioning as a load-balancer.
> 
> We reverse proxy to 6 nginx webservers running a number of Unicorn (Rails)
> application servers, these webserver nginx instances also run Evan Miller's
> mod_zip to assemble archives on the fly. We have discovered under certain
> circumstances the load-balancing nginx will "hang-up" on the webserver if
> the load-balancer is configured with proxy_buffering off, however
> proxy_buffering on seems to succeed. We would prefer to run without
> proxy_buffering to prevent the load-balancer's local storage from being
> overrun.

If you want to disable disk buffering you don't need to disable 
buffering at all.  Use

    proxy_max_temp_file_size 0;

instead.

The only valid reason to disable buffering completely with 
"proxy_buffering off" is when you need even single byte from 
backend to be immediately passed to client, i.e. as in some 
streaming / long polling cases.

> Our default setup uses nginx 0.7.65 for both the load-balancer and the
> webserver, however switching to using 1.0.12 as the load-balancer has the
> same problem. We have experimented with different software doing the
> load-balancing and it does not exhibit this issue.
> 
> I've have linked the nginx configuration file we're using on the load
> balancer, and debug logs for both 0.7.65 and 1.0.12.
> 
> https://x.onehub.com/transfers/sg32zsar
> 
> The buffering on log is very long, but it does show success of a 4.8GB
> response, the other responses always fail at the same point (826 MB).
> 
> The client sees the following (in access.log):
> 
> $ curl -b cookie.txt -o US.zip https://mydomain.com/folders/7816672/archive
>   % Total    % Received % Xferd  Average Speed   Time    Time     Time
>  Current
>                                  Dload  Upload   Total   Spent    Left
>  Speed
>  16 4922M   16  826M    0     0  1906k      0  0:44:03  0:07:23  0:36:40
> 1075k
> curl: (18) transfer closed with 4294967296 bytes remaining to read

The response is over 4G, and this won't work with "proxy_buffering 
off" in 1.0.x on 32bit systems.  The non-buffered mode was 
originally designed for small memcached responses and used to use 
size_t for length storage.

You have to upgrade to 1.1.x where it now uses off_t and will be 
able to handle large responses even on 32bit platforms.  
Alternatively, just forget about "proxy_buffering off" as you 
don't need it anyway, see above.

Maxim Dounin