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 <[email protected]> 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 > > > -- > dm-devel mailing > [email protected]https://www.redhat.com/mailman/listinfo/dm-devel > > >
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