On Thu, Dec 20, 2018 at 01:37:40PM +0800, Yi Zhang wrote: > 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.
In that case how about an interface where user tells QEMU "this backend is in persistent memory"? > > > > > > > > > > -- > > > > MST > > > > > >
