On Tue, Sep 26, Haripriya S wrote: > I had previously put out some performance numbers for origin writes > where performance goes down drastically w.r.t. the number of snapshots. > Going further, we identified one of the reasons for the performance drop > with increase in number of snapshots as the COW copies that happen to > every snapshot COW device when an origin write happens. Thanks a lot for your work in this area of the device-mapper. Your performance numbers show that work is really necessary here. > We have currently experimented with dm-snapshot code with two different > approaches and have got good performance numbers. I describe the first > approach and the results here and appreciate your opinions and inputs on > this. > > Approach 1 - Chained delta snapshots This means that every snapshot still has its own exception store. This would make deletion of snapshots unnecessary complex. It moves the work (copying of chunks) to the deletion of the snapshot. We discussed some of the ideas about snapshots here at the dm summit. The general ideas are as follows: - one exception store per origin device that is shared by all snapshots - don't keep the complete exception tables in memory all the time - limit kcopyd outstanding requests This would address the two biggest problems that I see with the snapshot target. The throughput issues should be addressed by only writing to one exception store. The memory issues should be addressed by the changes to the exception table handling. Although that includes a complete redesign of the exception store code. There are still ongoing discussions about the snapshot target. It would be nice if you have additional thoughts about this proposal. I guess it is similar to one of your prototypes. Jan -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
