From: Zhang Yi <yi.z.zh...@linux.intel.com> Signed-off-by: Zhang Yi <yi.z.zh...@linux.intel.com> --- 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..166c395 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 + error. + + - '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
