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

From: Yi Zhang <yi.z.zhang@linux.intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: peter.maydell@linaro.org, yu.c.zhang@linux.intel.com,
	ehabkost@redhat.com, imammedo@redhat.com, qemu-devel@nongnu.org,
	pbonzini@redhat.com, stefanha@redhat.com,
	crosthwaite.peter@gmail.com, richard.henderson@linaro.org,
	xiaoguangrong.eric@gmail.com
Subject: Re: [Qemu-devel] [PATCH V7 6/6] hostmem-file: add 'sync' option
Date: Thu, 20 Dec 2018 13:37:40 +0800	[thread overview]
Message-ID: <20181220053739.GB70591@tiger-server> (raw)
In-Reply-To: <20181219223700-mutt-send-email-mst@kernel.org>

On 2018-12-19 at 22:42:07 -0500, Michael S. Tsirkin wrote:
> 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.
> 
By default, we have already ignore the 2 flags, unless the administrator
know how to make that use. By-now, seems we don't have a better way to detect what
memory-backend-file is, a persistent memory or not.
> 
> > > 
> > > -- 
> > > MST
> > > 
>