[dm-devel] Question about merge performance

[Michael McCarthy]

I appreciate the insight. Are there any plans to enhance the code to allow
more parallel execution and/or allow users to experiment with QD?

Thanks, again.
Mike

On Thu, Mar 23, 2017 at 7:02 PM, Heinz Mauelshagen <heinzm at redhat.com>
wrote:

>
>
> 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 practically what it is.
>
> Snapshot merge is based on dm-kcopyd (device-mapper kernel copy daemon).
>
> The merge performs copies from the snapshots exception store to the origin
> LV sequentially.
> In each step It maximizes the payload per copy in case it identifies
> sequential origin chunks
> in consecutive entries in the snapshot exception store.
>
> The success of this optimization depends on previous update patterns on
> snapshot and origin:
> - the more sequential update patterns occured the better
> - with random update patterns the paylod per copy will drop to snapshot
> chunk_size worst case
>
> There's no knob to tweak this.
> The "snapshot-merge" target would need to be enhanced.
>
> 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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>
>
>
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