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?
