Re: [Qemu-devel] Why only devdax guarantees guest data persistence ?

[Stefan Hajnoczi]

On Fri, Feb 15, 2019 at 05:09:31PM +0000, bipin.tomar--- via Qemu-devel wrote:
> Text from "docs/nvdimm.txt" says:
> Guest Data Persistence
> ----------------------
> 
> Though QEMU supports multiple types of vNVDIMM backends on Linux,
> currently the only one that can guarantee the guest write persistence
> is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
> which all guest access do not involve any host-side kernel cache.
> 
> I think here "host-side kernel cache" imply "page cache". Why does fsdax NOT 
> have the same persistence guarantees as devdax for vNVDIMM?
> Both the modes avoid using page cache then why is devdax explicitly called 
> out?

File systems may require msync(2)/fsync(2) to guarantee persistence even
with DAX (just a cache flush instruction may not be enough!).  Emulated
NVDIMM devices lack an fsync interface so guests are unable to fsync the
host file system.

This is not an issue with devdax since there is no host file system.

virtio-pmem is an effort to add a paravirtualized fsync-style interface
and should solve this problem in the future:
https://lkml.org/lkml/2019/1/9/541

Stefan

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