Re: [Qemu-devel] [PATCH v3 3/3] pc-dimm: factor out address space logic into MemoryDevice code

[Pankaj Gupta]

> > > > > > >> +
> > > > > > >> +    memory_region_add_subregion(&hpms->mr, addr - hpms->base,
> > > > > > >> mr);
> > > > > > > missing vmstate registration?
> > > > > > 
> > > > > > Missed this one: To be called by the caller. Important because e.g.
> > > > > > for
> > > > > > virtio-pmem we don't want this (I assume :) ).
> > > > > if pmem isn't on shared storage, then We'd probably want to migrate
> > > > > it as well, otherwise target would experience data loss.
> > > > > Anyways, I'd just reat it as normal RAM in migration case
> > > > 
> > > > Main difference between RAM and pmem it acts like combination of RAM
> > > > and
> > > > disk.
> > > > Saying this, in normal use-case size would be 100 GB's - few TB's
> > > > range.
> > > > I am not sure we really want to migrate it for non-shared storage
> > > > use-case.
> > > with non shared storage you'd have to migrate it target host but
> > > with shared storage it might be possible to flush it and use directly
> > > from target host. That probably won't work right out of box and would
> > > need some sort of synchronization between src/dst hosts.
> > 
> > Shared storage should work out of the box.
> > Only thing is data in destination
> > host will be cache cold and existing pages in cache should be invalidated
> > first.
> > But if we migrate entire fake DAX RAMstate it will populate destination
> > host page
> > cache including pages while were idle in source host. This would
> > unnecessarily
> > create entropy in destination host.
> > 
> > To me this feature don't make much sense. Problem which we are solving is:
> > Efficiently use guest RAM.
> What would live migration handover flow look like in case of
> guest constantly dirting memory provided by virtio-pmem and
> and sometimes issuing async flush req along with it?

Dirty entire pmem (disk) at once not a usual scenario. Some part of disk/pmem
would get dirty and we need to handle that. I just want to say moving entire
pmem (disk) is not efficient solution because we are using this solution to
manage guest memory efficiently. Otherwise it will be like any block device copy
with non-shared storage.   
 
> 
> 
> > > The same applies to nv/pc-dimm as well, as backend file easily could be
> > > on pmem storage as well.
> > 
> > Are you saying backing file is in actual actual nvdimm hardware? we don't
> > need
> > emulation at all.
> depends on if file is on DAX filesystem, but your argument about
> migrating huge 100Gb- TB's range applies in this case as well.
> 
> > 
> > > 
> > > Maybe for now we should migrate everything so it would work in case of
> > > non shared NVDIMM on host. And then later add migration-less capability
> > > to all of them.
> > 
> > not sure I agree.
> So would you inhibit migration in case of non shared backend storage,
> to avoid loosing data since they aren't migrated?

I am just thinking what features we want to support with pmem. And live 
migration
with shared storage is the one which comes to my mind.

If live migration with non-shared storage is what we want to support (I don't 
know
yet) we can add this? Even with shared storage it would copy entire pmem state?

Thanks,
Pankaj
 
> 
> 
> > > > One reason why nvdimm added vmstate info could be: still there would be
> > > > transient
> > > > writes in memory with fake DAX and there is no way(till now) to flush
> > > > the
> > > > guest
> > > > writes. But with virtio-pmem we can flush such writes before migration
> > > > and
> > > > automatically
> > > > at destination host with shared disk we will have updated data.
> > > nvdimm has concept of flush address hint (may be not implemented in qemu
> > > yet)
> > > but it can flush. The only reason I'm buying into virtio-mem idea
> > > is that would allow async flush queues which would reduce number
> > > of vmexits.
> > 
> > Thats correct.
> > 
> > Thanks,
> > Pankaj
> > 
> >  
> 
> 
>