[DRBD-user] blocking I/O with drbd

Volker mail at blafoo.org
Mon Dec 19 17:08:55 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.


> I may be going blind, but I still fail to find the type of network
> adapter in use.
You're not blind, its not mentioned anywhere :-) Its a broadcom:

08:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5721
Gigabit Ethernet PCI Express (rev 21)

>> $ scp test-data.dd root at fallback:
>> test-data.dd       100%  392MB  39.2MB/s   00:10
> SCP isn't a very good test, because you get a lot of overhead for
> encryption. Still, this is way below what you seem to expect from your
> disk. How ist DRBD supposed to move more than 50MBps through your network?
I used scp despite the fact of the overhead because it already shows,
that the network-link is capable of way more than 25MB/s.

50MB/s? Where do you get that i expect that? DRBD is configured to a
maximum of 25MB/s.

What i do expect is that DRBD does a LOCAL speed of more than 1.5MB/s.
Please see my earlier emails doing dd with oflag=direct.

If i can only do 1.5MB/s locally, why should the network-link be the
Why does the load on the master go up while dd'ing, even if the
secondary is not attached?

And finally:
The overall average usage on the drbd-nic is about 1-2MB/s which is far
lower than the configured maximum of 25MB/s. The 25MB/s are only
saturated during a (re)sync and even the 25MB/s are far less than what
the link is capable of (even using scp).

It is not the network-link.

>> 64 bytes from fallback.content.domain.de.de ( icmp_seq=3
>> ttl=64 time=0.093 ms
> On a LAN, this is pretty serious latency. Don't expect DRBD to be
> whizzing fast in this environment.
> You *may* try and opt for protocol B to see if that helps latency, but
> you won't solve your throughput issues.
Im already using proto B and throughput is definatelly not the problem
here. if i can 'create' the problem locally without a secondary
attached, how can the network-link be of relevance?

- volker

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