From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:35618) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1goMjP-0003CZ-QW for qemu-devel@nongnu.org; Tue, 29 Jan 2019 01:18:50 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1goMjO-0006T5-Ee for qemu-devel@nongnu.org; Tue, 29 Jan 2019 01:18:47 -0500 Received: from mga09.intel.com ([134.134.136.24]:34449) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1goMjO-0006P9-6Z for qemu-devel@nongnu.org; Tue, 29 Jan 2019 01:18:46 -0500 From: "Zhang, Yi" Date: Tue, 29 Jan 2019 22:49:18 +0800 Message-Id: <4f91d5a46fc26d5672c91d80b78e986d4267c612.1548771590.git.yi.z.zhang@linux.intel.com> In-Reply-To: References: Subject: [Qemu-devel] [PATCH v11 3/3] 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 | 29 ++++++++++++++++++++++++++++- qemu-options.hx | 4 ++++ 2 files changed, 32 insertions(+), 1 deletion(-) diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt index 5f158a6..9da96aa 100644 --- a/docs/nvdimm.txt +++ b/docs/nvdimm.txt @@ -142,11 +142,38 @@ backend of vNVDIMM: Guest Data Persistence ---------------------- +vNVDIMM is designed and implemented to guarantee the guest data +persistence on the backends in case of host crash or a power 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 dax backend files with MAP_SYNC, which +ensures filesystem metadata consistency in case of a host crash or a power +failure. Enabling MAP_SYNC in QEMU requires below conditions + + - 'pmem' option of memory-backend-file is 'on': + The backend is a file supporting DAX, e.g., a file on an ext4 or + xfs file system mounted with '-o dax'. if your pmem=on ,but the backend is + not a file supporting DAX, mapping with this flag results in an EOPNOTSUPP + warning. then MAP_SYNC will be ignored + + - 'share' option of memory-backend-file is 'on': + MAP_SYNC flag available only with the MAP_SHARED_VALIDATE mapping type. + + - 'MAP_SYNC' is supported on linux kernel.(default opened since Linux 4.15) + +Otherwise, We will ignore the MAP_SYNC flag. + +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 guest NVDIMM region mapping structure. This unarmed flag indicates diff --git a/qemu-options.hx b/qemu-options.hx index 08f8516..0cd41f4 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} in case of host crash +or a power failure. MAP_SYNC requires support from both the host kernel +(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