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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