[DRBD-user] Limit Syncer Speed

Jasmin J. jasmin at anw.at
Thu Dec 8 12:00:20 CET 2016

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


I am using DRBD 8.4.

I want to limit the syncer speed to approx. 300MBit/s .

During intial sync after creating the disk this worked as expected. The
parameter controlling this was "c-max-rate".
But during normal operation DRBD seems to sync always with the fastest speed

I tried also protocol versions A and C and didn't see significant differences
concerning the used network bandwidth.

This is my currently used setup:

         disk {
                 # in units of 0.1 seconds
                 # 0,1s * 5 => 0,5s
                 c-plan-ahead 5;

                 # in units of 0.1 seconds
                 # 2 seconds max sync delay
                 c-delay-target 20;

                 # in Units of KiB/s
                 # ca. 300MBit/s
                 c-max-rate 35M;

                 # in Units of KiB/s
                 # 32MBit/s
                 c-min-rate 4M;

                 # will overrule c-delay-target, but seems to be better
                 # concerning linearity of the used network bandwith
                 c-fill-target 18M;

         net {
                 #protocol C;
                 max-buffers 8000;
                 max-epoch-size 8000;
                 sndbuf-size 512k;
                 cram-hmac-alg sha1;
                 shared-secret "XXXXX";
                 verify-alg md5;

                 protocol A;
                 on-congestion pull-ahead;
                 congestion-fill 2G;
                 congestion-extents 2000;

I commented "c-fill-target" -> no change
I also tried this (on both sides):
    drbdadm disk-options --c-plan-ahead=0 --c-max-rate=1M <resource>
  -> no change

I test the used network bandwidth with the tool
    speedometer -s -r eth0 -t eth0
It shows always 940MBit/s on a 1GBit ETH link, which is the whole bandwidth
of this link.

Is it possible, that the driver ignore this settings after initial sync?

Any ideas?


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