[DRBD-user] DRBD 8.3.7 and Debian 6 (2.6.32-5-xen-amd64)

Digimer lists at alteeve.ca
Tue Jan 28 15:56:02 CET 2014

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

On 28/01/14 09:42 AM, ml ml wrote:
> Hello List,
> i am running Debian 6 (2.6.32-5-xen-amd64) with DRBD 8.3.7
> I know - i should upgrade now - or better  => Yesterday :)
> I will, and i already did pre tests. However we currently have a
> performance problem with one of our production servers.
> They are connected with a intel GBit Nic. iperf schows 916 Mbits/sec in
> both directions.
> I have got SSDs on both sides. So I/O is not a problem.
> I only get a sync speed: 56,036 (53,756) K/sec
> Here is my config:
> =================
> global {       usage-count yes; }
> common {      syncer { rate 200M; } }

This is probably the problem. Setting it higher than is possible can 
actually hurt performance, if not stall out the sync entirely.

That said; You do understand that sync speed eats into replication speed 
and that sync and replication are two different things?

When you write data to your DRBD partition, it is replicated to the peer 
as fast as possible.

Sync only factors in when DRBD has to copy out-of-sync blocks to the 
peer in the background.

Assume you can normally replicate at 90 MB/sec, and you set sync to 80 
MB/sec... Now a node goes offline and 10 GB of data changes. When you 
reconnect the node, the sync will go at 80 MB/sec, leaving a mere 10 
MB/sec for you applications trying to actually use the DRBD partition, 
until the 10 GB of changed data is sync'ed.

Usually not what people want.

The rule of thumb is to set sync speed to no more than ~30% of *tested* 
maximum write speed. On a 1 Gbps link, I'd generally not set anything 
higher than syncer { rate: 30M; }.

And yes, upgrade! :)

Papers and Projects: https://alteeve.ca/w/
What if the cure for cancer is trapped in the mind of a person without 
access to education?

More information about the drbd-user mailing list