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Re: [Qemu-block] [PATCH v15 08/25] block: introduce auto-loading bitmaps

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v15 08/25] block: introduce auto-loading bitmaps
Date: Fri, 17 Feb 2017 17:54:16 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1

17.02.2017 17:24, Kevin Wolf wrote:
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().

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

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.


With open/close all is already ok - bitmaps will not be saved because of BDRV_O_INACTIVE and will not be loaded because of IN_USE.

Best regards,

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