[DRBD-user] Sensible maximum number of drbd devices

Ben bench at silentmedia.com
Fri Apr 7 03:42:28 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.

On Apr 6, 2006, at 1:35 AM, Michael Paesold wrote:

> Ben wrote:
>> For what it's worth, I'm doing exactly what you're looking to do  
>> right now with a whopping 7 DRBD devices. My write performance  
>> stinks - I can only sync at 4MB/sec before a CPU core becomes  
>> entirely busy in IO wait - but I suspect that's due to some  
>> misconfiguration, and haven't had much time to look into it.
> Thanks for the bit of information.
> 4MB/sec initial re-sync? Or 4MB/sec write performance in dom0 or in  
> domU? If its write performance in domU, you should look into  
> scheduling configuration.

Both resync and steady-state write performance. Unfortunately I don't  
have any drbd resources mounted in my dom0 domains, but scp claims  
sustained write performance from one dom0 to another is around 25MB/s  
over my gigabit link. More interestingly, I do have a domU that isn't  
backed by DRBD, and it also only sees about 4MB/s write speeds. So I  
suspect my slowness has something to do with Xen, and not DRBD.

> See for example this thread:
> http://lists.xensource.com/archives/html/xen-users/2006-04/ 
> msg00115.html
> Doing something like the mentioned 'xm sched-sedf 0 0 0 0 1 1'  
> could help.

Alas, this did not help, but at least it gives me a place to look for  
more information.

>> Other than the write performance, using DRBD and xen with  
>> heartbeat is really, really cool.
> Did you also try with less drbd devices and measure performance?

Nope. I suspect that drbd is capable of far more than my piddling  
little usage scenario, and I suspect that my performance issues have  
nothing at all do to it. I don't need more than I've configured right  
now (or rather I need more RAM before I configure more domUs) so I  
haven't tested with anything larger.

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