Am 17.02.2017 um 14:48 hat Denis V. Lunev geschrieben: > On 02/17/2017 04:34 PM, Kevin Wolf wrote: > > Am 17.02.2017 um 14:22 hat Denis V. Lunev geschrieben: > >> But for sure this is bad from the downtime point of view. > >> On migrate you will have to write to the image and re-read > >> it again on the target. This would be very slow. This will > >> not help for the migration with non-shared disk too. > >> > >> That is why we have specifically worked in a migration, > >> which for a good does not influence downtime at all now. > >> > >> With a write we are issuing several write requests + sync. > >> Our measurements shows that bdrv_drain could take around > >> a second on an averagely loaded conventional system, which > >> seems unacceptable addition to me. > > I'm not arguing against optimising migration, I fully agree with you. I > > just think that we should start with a correct if slow base version and > > then add optimisation to that, instead of starting with a broken base > > version and adding to that. > > > > Look, whether you do the expensive I/O on open/close and make that a > > slow operation or whether you do it on invalidate_cache/inactivate > > doesn't really make a difference in term of slowness because in general > > both operations are called exactly once. But it does make a difference > > in terms of correctness. > > > > Once you do the optimisation, of course, you'll skip writing those > > bitmaps that you transfer using a different channel, no matter whether > > you skip it in bdrv_close() or in bdrv_inactivate(). > > > > Kevin > I do not understand this point as in order to optimize this > we will have to create specific code path or option from > the migration code and keep this as an ugly kludge forever.
The point that I don't understand is why it makes any difference for the follow-up migration series whether the writeout is in bdrv_close() or bdrv_inactivate(). I don't really see the difference between the two from a migration POV; both need to be skipped if we transfer the bitmap using a different channel. Maybe I would see the reason if I could find the time to look at the migration patches first, but unfortunately I don't have this time at the moment. My point is just that generally we want to have a correctly working qemu after every single patch, and even more importantly after every series. As the migration series is separate from this, I don't think it's a good excuse for doing worse than we could easily do here. Kevin
