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List:       qemu-block
Subject:    Re: [Qemu-block] [PATCH v2 2/2] iotests: simple mirror test with kvm on 1G image
From:       Vladimir Sementsov-Ogievskiy <vsementsov () virtuozzo ! com>
Date:       2018-11-30 16:14:23
Message-ID: d7eb30d3-1bfa-fc1f-4653-06badc435a80 () virtuozzo ! com
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30.11.2018 18:33, Max Reitz wrote:
> On 30.11.18 16:24, Vladimir Sementsov-Ogievskiy wrote:
> > 30.11.2018 17:59, Max Reitz wrote:
> > > On 30.11.18 15:51, Vladimir Sementsov-Ogievskiy wrote:
> > > > 30.11.2018 17:10, Kevin Wolf wrote:
> > > > > Am 30.11.2018 um 14:48 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > > > > 30.11.2018 16:13, Max Reitz wrote:
> > > > > > > On 30.11.18 14:06, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > > > 30.11.2018 15:30, Max Reitz wrote:
> > > > > > > > > On 29.11.18 11:18, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > > > > > This test is broken without previous commit fixing dead-lock in \
> > > > > > > > > > mirror. 
> > > > > > > > > > Signed-off-by: Vladimir \
> > > > > > > > > >                 Sementsov-Ogievskiy<vsementsov@virtuozzo.com>
> > > > > > > > > > ---
> > > > > > > > > > tests/qemu-iotests/235     | 59 \
> > > > > > > > > > ++++++++++++++++++++++++++++++++++++++ tests/qemu-iotests/235.out \
> > > > > > > > > > |  1 + tests/qemu-iotests/group   |  1 +
> > > > > > > > > > 3 files changed, 61 insertions(+)
> > > > > > > > > > create mode 100755 tests/qemu-iotests/235
> > > > > > > > > > create mode 100644 tests/qemu-iotests/235.out
> > > > > > > > > I'll get to the first patch in a second, but first a suggestion for \
> > > > > > > > >                 this
> > > > > > > > > patch: I think it's not so good to use 2 GB of space for a test (1 \
> > > > > > > > > GB for the source, 1 GB for the target).  So I tried my luck and \
> > > > > > > > > found that the test works, too, if you just use \
> > > > > > > > > preallocation=metadata for the source (instead of actually writing \
> > > > > > > > > data) and blockdev-mirror'ing the data to a throttled null-co \
> > > > > > > > > device.
> > > > > > > > 
> > > > > > > > Hmm, so parsing metadata is enough for qcow2 to yield on write, yes?
> > > > > > > 
> > > > > > > Apparently so.  If you can confirm that applying those changes to the
> > > > > > > test still make it work (i.e., fail before patch 1, pass afterwards),
> > > > > > > then I think it is just as good.
> > > > > > 
> > > > > > Ok, I've checked that your changes works for me.
> > > > > > 
> > > > > > hm, but we write to null, so, yield on write comes from throttling, \
> > > > > > however, without preallocation=metadata, it don't work.., do you know, \
> > > > > > why we need preallocation to reproduce?
> > > > > 
> > > > > If I should take a guess, probably because mirror only copies allocated
> > > > > clusters?
> > > > > 
> > > > > Kevin
> > > > > 
> > > > 
> > > > hm, so, what preallocation=metadata does? I thought, it preallocates tables, \
> > > > but all qcow2 clusters would be unallocated..
> > > 
> > > No, it initializes all metadata structures (i.e., L2 tables) so they
> > > point to data clusters; which is why you cannot use it with backing
> > > files.  It's just that the data clusters are not written.
> > 
> > so, clusters are allocated? in this case, we don't avoid allocating of 1G, but \
> > only have a performance gain?
> 
> The data isn't written, so less than 1 MB are actually allocated on-disk.
> 
> The L2 tables do point to offsets over a range of 1 GB, but they should
> all be holes on the filesystem.
> 
> Max
> 

Understand, ok, thank you!


-- 
Best regards,
Vladimir


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