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Re: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation
From: |
Yi Zhang |
Subject: |
Re: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation |
Date: |
Fri, 8 Feb 2019 21:07:14 +0800 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On 2019-02-07 at 22:00:46 -0500, Michael S. Tsirkin wrote:
> On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote:
> > From: Zhang Yi <address@hidden>
> >
> > Signed-off-by: Zhang Yi <address@hidden>
> > ---
> > 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).
>
> So I would just drop above two lines.
>
> How does one know that there's support for direct access?
> it's just a question of kernel right?
>
> And I don't think it matter wrt "real NVDIMM" - e.g. it
> could be vNVDIMM in a nested virt scenario.
> It will still survive host (i.e. L1 guest) crash.
>
> Besides, the above two lines aren't grammatical.
Ah.. Reasonable, will drop these two lines.
Thanks for your kindly review. Michael.
>
> > +
> > +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,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave}
> >
> > --
> > 2.7.4
- [Qemu-devel] [PATCH V13 0/5] support MAP_SYNC for memory-backend-file, Zhang, Yi, 2019/02/07
- [Qemu-devel] [PATCH V13 1/5] util/mmap-alloc: Add a 'is_pmem' parameter to qemu_ram_mmap, Zhang, Yi, 2019/02/07
- [Qemu-devel] [PATCH V13 2/5] scripts/update-linux-headers: add linux/mman.h, Zhang, Yi, 2019/02/07
- [Qemu-devel] [PATCH V13 3/5] linux-headers: add linux/mman.h., Zhang, Yi, 2019/02/07
- [Qemu-devel] [PATCH V13 4/5] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap(), Zhang, Yi, 2019/02/07
- [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation, Zhang, Yi, 2019/02/07
- Re: [Qemu-devel] [PATCH V13 0/5] support MAP_SYNC for memory-backend-file, Michael S. Tsirkin, 2019/02/07
- Re: [Qemu-devel] [PATCH V13 0/5] support MAP_SYNC for memory-backend-file, Pankaj Gupta, 2019/02/07