[DRBD-user] drbd on top of md: high load on primary during secondary resync

Walter Haidinger walter.haidinger at gmx.at
Wed Feb 15 11:32:10 CET 2012

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


-------- Original-Nachricht --------

> I am using different (from yours) versions of drbd and kernel with similar
> symptoms, so it is not a bug and even expected behavior. By default raid
> is
> rebuild at the maximum possible speed and throttled down when there is IO,
> while drbd also tries to resync at its configured speed ... the load on
> the
> disks in this case may rise above their limits.

I see. To clarify: I was talking about system load (as shown by uptime
or top), not disk load.
 
> I have limited the resync speed for both drbd and the raid in general to
> some lower value than the disks are capable (and leave some for the apps
> too). Because in my case, drbd takes (much) less time to resync, when
> necessary i manually set the raid resync to the minimum possible value
> (may
> even try with 0 as speed_max). Another option is to pause-sync /
> resume-sync of drbd instead of disconnecting. With the above the load is
> still reaching 5-10, but at least the system stays responsive.

Good idea, thanks! I've limited drbd resync speed, but not for md.
I'll lower dev.raid.speed_limit_max and see if it helps.

Still, I'm curious why a raid resync on the secondary node drives
up the system load on the primary even while the drbd link is idle.
Maybe the devs can explain this briefly?

Best regards,
Walter
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