Re: [PATCH] docs/nvdimm: add 'pmem=on' for the device dax backend file

[Pankaj Gupta]

> At the end of live migration, QEMU uses msync() to flush the data to
> the backend storage. When the backend file is a character device dax,
> the pages explicitly avoid the page cache. It will return failure from 
> msync().
> The following warning is output.
>
>     "warning: qemu_ram_msync: failed to sync memory range“
>
> So we add 'pmem=on' to avoid calling msync(), use the QEMU command line:
>
>     -object memory-backend-file,id=mem1,pmem=on,mem-path=/dev/dax0.0,size=4G
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
> ---
>  docs/nvdimm.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index c2c6e441b3..31048aff5e 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -243,6 +243,13 @@ use the QEMU command line:
>
>      -object memory-backend-file,id=nv_mem,mem-path=/XXX/yyy,size=4G,pmem=on
>
> +At the end of live migration, QEMU uses msync() to flush the data to the
> +backend storage. When the backend file is a character device dax, the pages
> +explicitly avoid the page cache. It will return failure from msync().
> +So we add 'pmem=on' to avoid calling msync(), use the QEMU command line:
> +
> +    -object memory-backend-file,id=mem1,pmem=on,mem-path=/dev/dax0.0,size=4G
> +
>  References
>  ----------
>
> --
Good to document this.

Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>

> 2.17.1
>
>