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

Lionel Sausin ls at numerigraphe.com
Sat Mar 12 14:53:12 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


Dear DRBD community,

We anticipate that sometime we'll have big write activity spikes on our 
DRBD cluster, like someone writing a 100GB video work.
The network between the two main nodes is slow, so we may want to let 
the secondary node fall behind, and/or we may use drbd-proxy.
I gave the new congestion management 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?

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 would drbd-proxy do?

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?

Finally, could someone please shed some light on the 
"congestion-extents" parameter too? All the docs say is it has the same 
limit as al-extents, but what is it for? Is it mandatory?

Our setup is composed of 3 nodes with stacked resources.
The low-level resources are replicated between nodes A and B through a 
WiMax link capable of 5MBps.
The stacked resources are replicated to node S. A has a GigE connection 
to S, but B has to use the WiMax link to reach S.
The underlying RAID array can handle ~300MBps sustained writes.
The servers have 8GB RAM, 7GB of which is free.
I'll send the drbd config files if someone wants.

Thanks in advance to anyone who can help make this clearer.

Lionel Sausin - Numérigrahe SARL.




More information about the drbd-user mailing list