Re: [Qemu-devel] [Qemu-block] Some question about savem/qcow2 incremental snapshot

[Stefan Hajnoczi]

On Fri, Jun 08, 2018 at 05:02:58AM +0000, He, Junyan wrote:
> I use the simple way to handle this,
> 1. Separate the nvdimm region from ram when do snapshot.
> 2. If the first time, we dump all the nvdimm data the same as ram, and enable 
> dirty log trace
> for nvdimm kind region.
> 3. If not the first time, we find the previous snapshot point and add 
> reference to its clusters
> which is used to store nvdimm data. And this time, we just save dirty page 
> bitmap and dirty pages.
> Because the previous nvdimm data clusters is ref added, we do not need to 
> worry about its deleting.
> 
> I encounter a lot of problems:
> 1. Migration and snapshot logic is mixed and need to separate them for nvdimm.
> 2. Cluster has its alignment. When do snapshot, we just save data to disk 
> continuous. Because we
> need to add ref to cluster, we really need to consider the alignment. I just 
> use a little trick way 
> to padding some data to alignment now, and I think it is not a good way.
> 3. Dirty log trace may have some performance problem.
> 
> In theory, this manner can be used to handle all kind of huge memory 
> snapshot, we need to find the 
> balance between guest performance(Because of dirty log trace) and snapshot 
> saving time.

If the snapshots are placed on the NVDIMM then save/load times should be
shorter.  I'm not sure how practical that is since this approach may be
too expensive for users.

Stefan

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