[DRBD-user] Ahead/behind and drbd-proxy

["Lionel Sausin, de la part de l'é]

Please let me answer myself, in case others look for the same answers later.
The following is only assumptions however, so please somebody correct me 
if I'm wrong.

> Sometime we have big activity spikes, like someone writing a 100GB 
> video work.
> The network is slow so we may want to let the secondary node fall 
> behind, and we may use drbd-proxy
> I gave the new features in v8.3.10 a try without drbd-proxy, and I 
> humbly admit I don't get the whole picture, so please let me ask a few 
> questions.
>
> First, is drbd-proxy required if we want to use the new congestion 
> settings in v3.8.10?
 From what I understand, it is not required, but the congestion features 
are mostly useless without it.
> If it's not required, then I wonder if it would be useful in our 
> setup. With video data, compression won't help, and when I push large 
> files to the drbd resource, "free" tells me the overall buffers get as 
> large as 7GB. What more does drbd-proxy do?
The 7GB were actually not at all in DRBD's buffers, but in the kernel's 
dirty pages - we are used to having a very high vm.dirty_ratio on 
storage servers, to use lots of RAM as write cache.
 From what I can tell, drbd-proxy would provide the following bonus vs 
dirty pages:
- smoother behavior: insensitive to sync() and various kernel 
optimizations trying to keep the dirty page count low
- compression
> Another point that I don't get is how drbd uses the "congestion-fill" 
> parameter. If I set it low (like 1K), the secondary node falls behind 
> as soon as there is write activity, as I expected. But if I set it to 
> 1G and I rsync a 10GB file full of zeros to the FS on the resource, 
> the secondary node never falls behind and rsync slows down to the 
> network speed. Am I doing it wrong?
Actually the TCP buffer never gets bigger than a few MB, so my guess is 
that any value of congestion-fill bigger than that will never make the 
secondary node fall behind.
I don't see the point in allowing values as big as 10GB, so I may still 
be missing something.

Lionel Sausin - Numérigrahe SARL.