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Re: [Qemu-block] [PATCH for-2.9-rc5 v4 2/2] block: Drain BH in bdrv_drai

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH for-2.9-rc5 v4 2/2] block: Drain BH in bdrv_drained_begin
Date: Wed, 7 Feb 2018 14:10:14 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 07.02.2018 um 13:39 hat Fam Zheng geschrieben:
> On Wed, Feb 7, 2018 at 6:51 PM, Kevin Wolf <address@hidden> wrote:
> > Am 07.02.2018 um 02:48 hat Fam Zheng geschrieben:
> >> On Tue, Feb 6, 2018 at 11:32 PM, Kevin Wolf <address@hidden> wrote:
> >> > I got a bug assigned where we have a large (200 GB) fragmented qcow2
> >> > image, and qemu-img convert takes two hours before it even starts to
> >> > copy data. What it does in this time is iterating through the whole
> >> > image with bdrv_block_status_above() in order to estimate the work to be
> >> > done (and of course it will repeat the same calls while actually copying
> >> > data).
> >>
> >> The convert_iteration_sectors loop looks wasteful. Why cannot we
> >> report progress simply based on (offset/size) * 100% so we don't need
> >> to do this estimation?
> >
> > The assumption is that it's quick and it makes the progress much more
> > meaningful. You know those progress bars that slowly crawl towards 50%
> > and then suddenly complete within a second. Or the first 20% are quick,
> > but then things get really slow. They are not a great example.
> >
> > There must have been something wrong with that image file that was
> > reported, because they reported that it was the only image causing
> > trouble and if they copied it away, it became quick, too.
> >
> > Even for a maximally fragmented 100 GB qcow2 image it only takes about a
> > second on my laptop. So while I don't feel as certain about the loop as
> > before, in practice it normally doesn't seem to hurt.
> No doubt about normal cases. I was unsure about corner cases like
> slow-ish NFS etc.

Yeah, NFS seems to be a bit slow. Not two-hours-slow, but when I tried
it with a localhost NFS server, the same operation that took two seconds
directly from the local file system took about 40s over NFS. They seem
to go over the network for each lseek() instead of caching the file map.
Maybe something to fix in the NFS kernel driver.

> A little bit of intelligence would be limiting the time for the loop
> to a few seconds, for example, "IMG_CVT_EST_SCALE *
> bdrv_getlength(bs)", or a less linear map.

I don't understand. What would IMG_CVT_EST_SCALE be? Isn't the problem
that this isn't a constant factor but can be anywhere between 0% and
100% depending on the specific image?

> >> > One of the things I saw is that between each pair of lseek() calls, we
> >> > also have unnecessary poll() calls, and these were introduced by this
> >> > patch. If I modify bdrv_common_block_status_above() so that it doesn't
> >> > poll if data.done == true already, the time needed to iterate through my
> >> > test image is cut in half.
> >> >
> >> > Now, of course, I'm still only seeing a few seconds for a 100 GB image,
> >> > so there must be more that is wrong for the reporter, but it suggests to
> >> > me that BDRV_POLL_WHILE() shouldn't be polling unconditionally when only
> >> > one of its users actually needs this.
> >>
> >> Sounds fine to me. Maybe we could add a boolean parameter to
> >
> > Why not call aio_poll() explicitly in the BDRV_POLL_WHILE() condition in
> > the one place that needs it?
> Yes, that is better. Do you have a patch? Or do you want me to work on one?

I don't have a patch yet. If you want to write one, be my guest.

Otherwise I'd just take a to-do note for when I get back to my
bdrv_drain work. There is still one or two series left to do anyway, and
I think it would fit in there.


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