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Re: [Qemu-block] [Qemu-devel] [PATCH 4/6] dirty-bitmaps: clean-up bitmap

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 4/6] dirty-bitmaps: clean-up bitmaps loading and migration logic
Date: Fri, 3 Aug 2018 09:33:44 +0100
User-agent: Mutt/1.10.0 (2018-05-17)

* Denis V. Lunev (address@hidden) wrote:
> On 08/02/2018 12:50 PM, Dr. David Alan Gilbert wrote:
> > * Denis V. Lunev (address@hidden) wrote:
> >
> >
> >>> I don't quite understand the last two paragraphs.
> >> we are thinking right now to eliminate delay on regular IO
> >> for migration. There is some thoughts and internal work in
> >> progress. That is why I am worrying.
> > What downtime are you typicaly seeing and what are you aiming for?
> >
> > It would be good if you could explain what you're planning to
> > fix there so we can get a feel for it nearer the start of it
> > rather than at the end of the reviewing!
> >
> > Dave
> The ultimate goal is to reliable reach 100 ms with ongoing IO and
> you are perfectly correct about reviewing :)

That would be neat.

> Though the problem is that right now we are just trying to
> invent something suitable :(

OK, some brain-storm level ideas:

  a) Throttle the write bandwidth at later stages of migration
     (I think that's been suggested before)
  b) Switch to some active-sync like behaviour where the writes
     are sent over the network as they happen to the destination
     (mreitz has some prototype code for that type of behaviour
     for postcopy)
  c) Write the writes into a buffer that gets migrated over the
    migration stream to get committed on the destination side.

As I say, brainstorm level ideas only!


> Den
> >>> However, coming back to my question; it was really saying that
> >>> normal guest IO during the end of the migration will cause
> >>> a delay; I'm expecting that to be fairly unrelated to the size
> >>> of the disk; more to do with workload; so I guess in your case
> >>> the worry is the case of big large disks giving big large
> >>> bitmaps.
> >> exactly!
> >>
> >> Den
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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