[DRBD-user] Raid controller cache module gives DRBD problems

Mark Hunting mark at netexpo.nl
Thu Dec 13 14:00:05 CET 2007

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

Hi all,

I have set up DRBD (0.7.21, Debian Etch) on two storage systems. Both
systems use a Areca Raid controller with 1GB cache memory. When I copy a
file to the DRBD device on the primary, the copying finishes with a
speed of about 400MB per second. That's should be impossible, DRBD uses
a gigabit link to synchronize, the copying should proceed with a rate op
100 MB per second (drbd.conf: rate 100M).

So apparently the copying finishes before the data is transferred to the
secondary. Indeed I still see a lot of DRBD network traffic for some
seconds after the copy command finishes. This should be impossible with
DRBD, the copying should only finish when the data is both written on
the primary AND the secondary, right? When the primary server
unexpectedly goes down right after a file has been copied and the
secondary server takes over, the copied file is not there.
I think this has to do with the 1GB cache memory on the Areca cards,
when I transfer files that are much bigger than 1GB the copying indeed
finishes with a speed close to 100MB per second.

I don't understand this, what difference does it make for DRBD what is
underneath the DRBD device (/dev/drbd0)? It should not matter if there
is only one hard disk, a RAID system, a RAID system with cache memory or
whatever. But apparently it does matter in my case.

Is this a known problem, and how can I avoid it? Right now I can never
be sure my two storage systems are fully synchronized.

Thanks a lot

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