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Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps |
Date: |
Fri, 17 Feb 2017 15:24:03 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
- [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, (continued)
- [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/15
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/16
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/16
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Denis V. Lunev, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Denis V. Lunev, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps,
Kevin Wolf <=
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/17
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Denis V. Lunev, 2017/02/18
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/20
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Denis V. Lunev, 2017/02/20
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/20
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Kevin Wolf, 2017/02/20
- Re: [Qemu-devel] [PATCH v15 08/25] block: introduce auto-loading bitmaps, Vladimir Sementsov-Ogievskiy, 2017/02/20
[Qemu-devel] [PATCH v15 02/25] specs/qcow2: do not use wording 'bitmap header', Vladimir Sementsov-Ogievskiy, 2017/02/15
[Qemu-devel] [PATCH v15 12/25] block/dirty-bitmap: add bdrv_dirty_bitmap_next(), Vladimir Sementsov-Ogievskiy, 2017/02/15
[Qemu-devel] [PATCH v15 04/25] tests: add hbitmap iter test, Vladimir Sementsov-Ogievskiy, 2017/02/15