[Xen-devel] Capping i/o ops

[Pim van Riezen <pi+lists@xxxxxxxxxxxx>]

Hi All,

I've been trying to get a bit of a grip on disk i/o for our Xen set- 
up. We're dealing with server workloads that can, at unexpected  
times, become seriously io-bound, up to the point that a single guest  
can cannibalize the available throughput of the fibrechannel array.  
Since we're dealing with a shared medium and multiple Xen hosts, I'm  
looking for a way in to put an upper cap on the number of i/o  
operations individual guests can perform.
Although this is not entirely scientific, it would allow an informed  
administrator to limit the amount of operations guest A on host X can  
perform to e.g., 50% of an observed maximum, so that guest B on host  
Y is still guaranteed access at a minimum performance level. The case  
could also be made for giving quality of service assurances on the  
single host level, but the multiple host scenario is the more  
interesting viewpoint: It precludes solutions that entail automatic  
optimizations - there are no trivial automatic solutions to be had in  
this scenario.
I'm not afraid to get my hands dirty and come up with a proof of  
concept or even completer patch, but before I dive in, would this  
approach stand a decent chance at making sense? The current approach  
seems to be "throw the requests at the block layer and let the  
kernel, the hardware, or God sort it out". Has any previous thought  
been given to this problem area that I can/should read up about?
Cheers,
Pim van Riezen



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