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

[Yi Zhang]

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.

> 
> -- 
> MST
>