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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