[virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 16, 2023 at 04:26:53PM +0000, Parav Pandit wrote:
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, November 16, 2023 5:18 PM
> > 
> > On Thu, Nov 16, 2023 at 07:40:57AM +0000, Parav Pandit wrote:
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Thursday, November 16, 2023 1:06 PM
> > > >
> > > > On Thu, Nov 16, 2023 at 12:51:40AM -0500, Michael S. Tsirkin wrote:
> > > > > On Thu, Nov 16, 2023 at 05:29:54AM +0000, Parav Pandit wrote:
> > > > > > We should expose a limit of the device in the proposed
> > > > WRITE_RECORD_CAP_QUERY command, that how much range it can track.
> > > > > > So that future provisioning framework can use it.
> > > > > >
> > > > > > I will cover this in v5 early next week.
> > > > >
> > > > > I do worry about how this can even work though. If you want a
> > > > > generic device you do not get to dictate how much memory VM has.
> > > > >
> > > > > Aren't we talking bit per page? With 1TByte of memory to track ->
> > > > > 256Gbit -> 32Gbit -> 8Gbyte per VF?
> > > >
> > > > Ugh. Actually of course:
> > > > With 1TByte of memory to track -> 256Mbit -> 32Mbit -> 8Mbyte per VF
> > > >
> > > > 8Gbyte per *PF* with 1K VFs.
> > > >
> > > Device may not maintain as a bitmap.
> > 
> > However you maintain it, there's 256Mega bit of information.
> There may be other data structures that device may deploy as for example hash or tree or something else.

Point being?

> And this is runtime memory only during the short live migration period of 400msec or less.
> It is not some _always_ resident memory.

No - write tracking is used in the live phase of migration. It can be
enabled as long as you wish - it's a question of policy.  There actually
exist solutions that utilize this phase for redundancy, permanently
running in this mode.

-- 
MST


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