Re: [Qemu-devel] [PATCH V7 6/6] hostmem-file: add 'sync' option

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 20, 2018 at 11:03:12AM +0800, Yi Zhang wrote:
> On 2018-12-19 at 10:59:10 -0500, Michael S. Tsirkin wrote:
> > On Wed, Dec 19, 2018 at 05:10:18PM +0800, Yi Zhang wrote:
> > > > > +
> > > > > + - 'sync' option of memory-backend-file is not 'off', and
> > > > > +
> > > > > + - 'share' option of memory-backend-file is 'on'.
> > > > > +
> > > > > + - 'pmem' option of memory-backend-file is 'on'
> > > > > +
> > > > 
> > > > Wait isn't this what pmem was supposed to do?
> > > > Doesn't it mean "persistent memory"?
> > > pmem is a option for memory-backend-file, user should know the backend
> > > is in host persistent memory, with this flags on, while there is a host crash
> > > or a power failures.
> > > 
> > > [1] Qemu will take necessary operations to guarantee the persistence.
> > > https://patchwork.ozlabs.org/cover/944749/ 
> > > 
> > > [2] Host kernel also take opretions to consistent filesystem metadata.
> > > Add MAP_SYNC flags.
> > 
> > OK so I'm a user. Can you educate me please?  
> We suppose an administrator should know it, what is the back-end region coming from,
> is it persistent? what is the font-end device is? a volatile dimm or an
> nonvolatile dimm? then they can choice put the pmem=on[off] and sync=on[off].
> If he didn't, we encourage that don't set these 2 flags.
> 
> > When should MAP_SYNC not
> > be set? Are there any disadvantages (e.g.  performance?)?
> Not only the performance, sometimes like the front-end device is an
> volatile ram, we don't wanna set such option although the backend is a
> novolatile memory, if power lose, all of thing should lose in this ram.



I am not sure how does above answer the questions. If I don't know,
neither will the hypothetical administrator. Looks like a better
interface is needed to make the choice on behalf of the user.


> > 
> > -- 
> > MST
> >