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

Lars Ellenberg lars.ellenberg at linbit.com
Thu Dec 13 17:16:48 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.

On Thu, Dec 13, 2007 at 02:00:05PM +0100, Mark Hunting wrote:
> 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.

man fsync
man fdatasync
man sync

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