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

[Michael S. Tsirkin]

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?  When should MAP_SYNC not
be set? Are there any disadvantages (e.g.  performance?)?

-- 
MST