On Thu, 20 Oct 2016 13:00:29 -0200
Eduardo Habkost <ehabk...@redhat.com> wrote:
> On Thu, Oct 20, 2016 at 04:18:20PM +0200, Igor Mammedov wrote:
> > On Thu, 20 Oct 2016 11:57:11 -0200
> > Eduardo Habkost <ehabk...@redhat.com> wrote:
> >
> > > On Thu, Oct 20, 2016 at 03:47:32PM +0200, Igor Mammedov wrote:
> > > > On Thu, 20 Oct 2016 21:33:53 +0800
> > > > Haozhong Zhang <haozhong.zh...@intel.com> wrote:
> > > >
> > > > > On 10/20/16 11:21 -0200, Eduardo Habkost wrote:
> > > > > >On Thu, Oct 20, 2016 at 02:34:12PM +0200, Igor Mammedov wrote:
> > > > > >> On Thu, 20 Oct 2016 14:13:01 +0800
> > > > > >> Haozhong Zhang <haozhong.zh...@intel.com> wrote:
> > > > > >>
> > > > > >> > If a file is used as the backend of memory-backend-file and its
> > > > > >> > size is
> > > > > >> > not identical to the property 'size', the file will be
> > > > > >> > truncated. For a
> > > > > >> > file used as the backend of vNVDIMM, its data is expected to be
> > > > > >> > persistent and the truncation may corrupt the existing data.
> > > > > >> >
> > > > > >> I wonder if it's possible just skip 'size' property in your case
> > > > > >> instead
> > > > > >> 'notrunc' property. That way if size is not present one'd get
> > > > > >> actual size
> > > > > >> using get_file_size() and set 'size' to it?
> > > > > >> And if 'size' is provided and 'size' != file_size then error out.
> > > > > >>
> > > > > >
> > > > > >I think it is valid to start with a zero-size file and then let
> > > > > >QEMU extend it.
> > > > >
> > > > > For vNVDIMM, extending from zero-size file can be valid when a file is
> > > > > first used. However, it's not valid for the second and following use
> > > > > of the same file.
> > > > I'd avoid 0 sized backend files and enforce non 0 size value
> > > > with exact match to actual file size. i.e. let mgmt side take care of
> > > > proper backend file allocation.
> > >
> > > This would break compatibility with existing setups that rely on
> > > the ftruncate() behavior.
> >
> > Question is what exactly rely on truncate behavior?
>
> hugetlbfs or tmpfs + mempolicy would work only if a sparse file
> used (otherwise memory would be already allocated before running
> QEMU). That means it is easier to simply not set file size before
> running QEMU.
>
> I don't know what's libvirt behavior, but I wouldn't want to
> break existing scripts that rely on it.
We can probably do something like this
if (file_size == 0 && explicitly_asked_size > file_size)
do allocation
