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Date: Thu, 20 Oct 2016 15:54:50 +0200
From: Igor Mammedov <imammedo@redhat.com>
Message-ID: <20161020155450.65353e12@nial.brq.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to
 avoid data corruption
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Haozhong Zhang <haozhong.zhang@intel.com>, kwolf@redhat.com, Xiao Guangrong <guangrong.xiao@linux.intel.com>, Eduardo Habkost <ehabkost@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, qemu-devel@nongnu.org, mreitz@redhat.com, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

On Thu, 20 Oct 2016 14:27:06 +0100
"Daniel P. Berrange" <berrange@redhat.com> 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.zhang@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.  
> 
> That works if you always want to expose the entire file, but if you
> intentionally only want to expose a subset you would still want
> to set a size (and possibly offset too) and avoid the truncation
we don't do it now and probably shouldn't do it at all to keep interface sane.
Idea of file backend has been simple:
 here if file to map and let frontend to use/partition it as it deems fit.

in case of NVDIMM frontend might partition backend file into data
and label areas (as it's implemented now).

> 
> 
> Regards,
> Daniel