[dm-devel] Question about merge performance

[Michael McCarthy]

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.

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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