On Wed, Jan 16, 2019 at 04:11:16PM +0800, Zhang Yi wrote:
> Signed-off-by: Zhang Yi <[email protected]>
> ---
> docs/nvdimm.txt | 21 ++++++++++++++++++++-
> qemu-options.hx | 4 ++++
> 2 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index 5f158a6..565ba73 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -142,11 +142,30 @@ backend of vNVDIMM:
> Guest Data Persistence
> ----------------------
>
> +vNVDIMM is designed and implemented to guarantee the guest data
> +persistence on the backends even on the host crash and power
in case of a host crash or a power failure
(be consistent)
> +failures. However, there are still some requirements and limitations
> +as explained below.
> +
> Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one that can guarantee the guest write persistence
> +if MAP_SYNC is not supported by the host kernel and the backends,
> +the only backend 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.
>
> +mmap(2) flag MAP_SYNC is added since Linux kernel 4.15. On such
> +systems, QEMU can mmap(2) the backend with MAP_SYNC, which can ensure
which ensures
> +filesystem metadata consistent
consistency
> even after a system crash or power
a power
> +failure. Besides the host kernel support, enabling MAP_SYNC in QEMU
> +also requires:
> +
> + - the backend is a file supporting DAX, e.g., a file on an ext4 or
> + xfs file system mounted with '-o dax',
> +
> + - 'share' option of memory-backend-file is 'on'.
why does share need to be on?
> +
> + - 'pmem' option of memory-backend-file is 'on'
> +
> When using other types of backends, it's suggested to set 'unarmed'
> option of '-device nvdimm' to 'on', which sets the unarmed flag of the
> guest NVDIMM region mapping structure. This unarmed flag indicates
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 08f8516..545cb8a 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4002,6 +4002,10 @@ using the SNIA NVM programming model (e.g. Intel
> NVDIMM).
> If @option{pmem} is set to 'on', QEMU will take necessary operations to
> guarantee the persistence of its own writes to @option{mem-path}
> (e.g. in vNVDIMM label emulation and live migration).
> +Also, we will map the backend-file with MAP_SYNC flag, which can ensure
> +the file metadata is in sync to @option{mem-path} even on the host crash
> +and power failures. MAP_SYNC requires supports from both the host kernel
requires support
> +(since Linux kernel 4.15) and @option{mem-path} (only files supporting DAX).
>
> @item -object
> memory-backend-ram,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave}
>
> --
> 2.7.4
