Summary from a private comment:
Description of problem:
Currently, when a snapshot is open with 'cache=none', the backing files are also opened without caching. However, as they are read-only, they can be cached. The downside is that it may cause duplication of caching - both in the host memory as well as the guest's. The upside is that if many VMs share the same backing file (as happens in VDI scenarios), this will make launching them much more efficient.
I think a configuration option allowing this would be helpful for VDI scenarios.
--- Additional comment from Kevin Wolf on 2016-02-26 10:46:25 BRT ---
Upstream commit 91a097e7 implements setting the cache modes for child nodes,
including the backing file. blockdev-add isn't a supported interface yet, but this can also be used with -drive and HMP drive_add.
Comment 4Jaroslav Suchanek
2019-04-24 12:26:51 UTC
This bug is going to be addressed in next major release.
Comment 7Jaroslav Suchanek
2020-11-12 16:58:22 UTC
This bug was closed deferred as a result of bug triage.
Please reopen if you disagree and provide justification why this bug should
get enough priority. Most important would be information about impact on
customer or layered product. Please indicate requested target release.