From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:60787) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1grvA4-0000oE-65 for qemu-devel@nongnu.org; Thu, 07 Feb 2019 20:41:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1grvA2-00057O-OV for qemu-devel@nongnu.org; Thu, 07 Feb 2019 20:41:00 -0500 Received: from mga07.intel.com ([134.134.136.100]:7015) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1grvA2-00050U-FR for qemu-devel@nongnu.org; Thu, 07 Feb 2019 20:40:58 -0500 From: "Zhang, Yi" Date: Fri, 8 Feb 2019 18:11:23 +0800 Message-Id: <42dd91019c7aa480cb0d26565fefcb1238c37dc8.1549555521.git.yi.z.zhang@linux.intel.com> In-Reply-To: References: Subject: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: xiaoguangrong.eric@gmail.com, stefanha@redhat.com, pbonzini@redhat.com, pagupta@redhat.com, yu.c.zhang@linux.intel.com, richardw.yang@linux.intel.com, mst@redhat.com, ehabkost@redhat.com Cc: qemu-devel@nongnu.org, imammedo@redhat.com, dan.j.williams@intel.com, Zhang Yi From: Zhang Yi Signed-off-by: Zhang Yi --- docs/nvdimm.txt | 25 ++++++++++++++++++++++--- qemu-options.hx | 5 +++++ 2 files changed, 27 insertions(+), 3 deletions(-) diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt index 5f158a6..a168429 100644 --- a/docs/nvdimm.txt +++ b/docs/nvdimm.txt @@ -143,9 +143,28 @@ 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. +the only backend that can guarantee the guest write persistence is: + +A. DAX device (e.g., /dev/dax0.0, ) or +B. DAX file(mounted with dax option) + +both are use real NVDIMM device as backend, which supporting direct +access for files(no page cache). + +When using B (A file supporting direct mapping of persistent memory) +as a backend, write persistence is guaranteed if the host kernel has +support for the MAP_SYNC flag in the mmap system call (available +since Linux 4.15 and on certain distro kernels) and additionally +both 'pmem' and 'share' flags are set to 'on' on the backend. + +If these conditions are not satisfied i.e. if either 'pmem' or 'share' +are not set, if the backend file does not support DAX or if MAP_SYNC +is not supported by the host kernel, write persistence is not +guaranteed after a system crash. For compatibility reasons, these +conditions are silently ignored if not satisfied. Currently, no way +is provided to test for them. +For more details, please reference mmap(2) man page: +http://man7.org/linux/man-pages/man2/mmap.2.html. 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 diff --git a/qemu-options.hx b/qemu-options.hx index 08f8516..ef1da8f 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4002,6 +4002,11 @@ 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 ensures the +file metadata is in sync for @option{mem-path} in case of host crash +or a power failure. MAP_SYNC requires support from both the host kernel +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted +with DAX option. @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