From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:47240) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gjof4-0005ZL-07 for qemu-devel@nongnu.org; Wed, 16 Jan 2019 12:07:32 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gjnJ0-0003dA-DO for qemu-devel@nongnu.org; Wed, 16 Jan 2019 10:40:39 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57752) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gjnIv-0003Zn-Qu for qemu-devel@nongnu.org; Wed, 16 Jan 2019 10:40:34 -0500 Date: Wed, 16 Jan 2019 10:40:23 -0500 From: "Michael S. Tsirkin" Message-ID: <20190116103615-mutt-send-email-mst@kernel.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH V9 6/6] docs: Added MAP_SYNC documentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Zhang Yi Cc: xiaoguangrong.eric@gmail.com, stefanha@redhat.com, pbonzini@redhat.com, pagupta@redhat.com, yu.c.zhang@linux.intel.com, ehabkost@redhat.com, qemu-devel@nongnu.org, imammedo@redhat.com, dan.j.williams@intel.com On Wed, Jan 16, 2019 at 04:11:16PM +0800, Zhang Yi wrote: > Signed-off-by: Zhang Yi > --- > 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