RE: [Xen-devel] high I/O cost for drbd and xen?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> Just run some benchmarks on my new failover cluster and found some
> alarming results. I've previously observed that xen typically 
> introduces
> almost zero IO overhead. I ran some tests with bonnie -x 10 on my drbd
> mirrors and compared the averages between dom0 and dom1 to see the
> virtualisation cost. I was surprised (and alarmed) to see 
> that there was
> a fairly high cost for read operations (high being >10%). I was very
> concerned to see the 80% cost for get_block on the failover node.

Not sure what's going on here as I've never used drbd.

I'd be inclined to run drbd in domain 0 and export the device to the
guest rather than run drbd in the guest.

Ian
  
> Could this be a scheduler issue? drbd likes to be high 
> priority - it's a
> high availability service after all. Perhaps the hypervisor 
> is blocking
> it
> or not allowing it its normal share of cpu time? Perhaps its 
> giving CPU
> time from drbd on dom0 to bonnie on dom1, causing the 
> reported slowdown?
>  
> Tom
>  
>  
> 


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