[DRBD-user] Sensible maximum number of drbd devices

Michael Paesold michael at paesold.at
Wed Apr 5 16:36:10 CEST 2006

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 am again testing drbd for use with xen virtual machines. On a typical 
physical pair of machines, there will be about 8 to 15 (maybe even 20) 
virtual machines.

Each VM needs its own block device for storage. To be able to fail over or 
migrate single VMs between the nodes, I must be able independently activate 
the block devices on one of the two nodes.

That raises the question of how many drbd devices one can reasonably use on 
a single cluster of two nodes without too much overhead. I just guess that 
in-kernel locking and synchronization, and the number of kernel threads 
could be an issue with a higher number of devices (I am no expert here). Are 
15 to 20 devices a reasonable number?

There are other options. For example drbd v0.8/8.0 (?) with primary/primary 
in combination with CLVM [1] would allow for a single shared drbd device 
with logical volumes for the VMs. Of course, a single logical volume would 
be accessed from only one host at a time, but the drbd device itself would 
be active on both nodes.

Does anybody have experience with DRBD and RedHat Cluster Suite integration?

Another option, if DRBD and CS do not play well together, would be a custom 
frontend for device-mapper to create logical volumes, but integrated with 
heartbeat and more aware of the underlying drbd.

So the basic question is still how many drbd devices one can reasonably use.

Best Regards,
Michael Paesold

[1] Cluster LVM; a cluster-aware metadata management for LVM, part of RedHat 
GFS/Cluster Suite 

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