[dm-devel] Question about merge performance

[Heinz Mauelshagen]

On 03/23/2017 05:35 PM, Michael McCarthy wrote:
> Hello, collective wisdom,
>
> I'm new to the list and apologize if this is not the right place to 
> ask this type of question and if so, would be glad receive pointers to 
> the correct one.
>
> Now the question:
>
> I'm looking for ways to improve snapshot-merge target performance. 
> We're using CentOS 7.3 here. Both, the snapshot-origin and the 
> snapshot (cow data holder) reside on NVMe SSDs. What we've seen in our 
> tests is that the speed of the merge isn't approaching neither the 
> throughput nor the IOPS limits of the NVMe devices. I suspect it might 
> be because the merge operation is single threaded and uses QD of 1.

Yes, that's what it is.

Using dm-kcopyd the snapshot merge sequentially copies exception store 
chunks across to the origin.

Though it aims to copy consecutive chunks across, the success of that 
optimization depends on
sequential write patterns to the origin and the snapshot populating the 
exception store.
In case writes were random, one chunk will be transferred per copy worst 
case.

There's no knob to tune this (but io schedulers which ain't help with 
yur NVMe backends).

The "snaphsot-merge" target would need to be enhanced to provide higher 
throughput.

Heinz

> Could anyone with enough knowledge about the DM code shed some light 
> on how it operates during the merge? Are there any interfaces to 
> improve the speed of this operation without altering the code?
>
> Thanks,
> Mike
>
>
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