[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 does.

Using dm-kcopyd snapshot merging submits payloads to be copied across to 
the origin
sequentially aiming to maximize individual payloads by identifying any 
consecutive exception
store chunks which are in origin adress order.

If your update patters to the snapshot have been sequential that 
optimization will help througput,
if those were random it doesn't and the payloads will be snapshot chunks 
in size.

There's no knob to tune this (but io scheduler which don't help with 
your NVMe storage)

The "snapshot-merge" target would need to be enhanced to provide better 
throughput.

Regards,
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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