Re: [Qemu-devel] [PATCH V5] qemu-img: align result of is_allocated_sectors

[Kevin Wolf <kwolf@redhat.com>]

Am 12.07.2018 um 11:26 hat Peter Lieven geschrieben:
> Am 11.07.2018 um 10:25 schrieb Kevin Wolf:
> > Am 10.07.2018 um 22:16 hat Peter Lieven geschrieben:
> > > 
> > > > Am 10.07.2018 um 17:31 schrieb Kevin Wolf <kwolf@redhat.com>:
> > > > 
> > > > Am 10.07.2018 um 17:05 hat Peter Lieven geschrieben:
> > > > > We currently don't enforce that the sparse segments we detect during convert are
> > > > > aligned. This leads to unnecessary and costly read-modify-write cycles either
> > > > > internally in Qemu or in the background on the storage device as nearly all
> > > > > modern filesystems or hardware have a 4k alignment internally.
> > > > > 
> > > > > This patch modifies is_allocated_sectors so that its *pnum result will always
> > > > > end at an alignment boundary. This way all requests will end at an alignment
> > > > > boundary. The start of all requests will also be aligned as long as the results
> > > > > of get_block_status do not lead to an unaligned offset.
> > > > > 
> > > > > The number of RMW cycles when converting an example image [1] to a raw device that
> > > > > has 4k sector size is about 4600 4k read requests to perform a total of about 15000
> > > > > write requests. With this path the additional 4600 read requests are eliminated while
> > > > > the number of total write requests stays constant.
> > > > > 
> > > > > [1] https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-amd64-disk1.vmdk
> > > > > 
> > > > > Signed-off-by: Peter Lieven <pl@kamp.de>
> > > > It looked convincing, but I'm afraid this is still not correct.
> > > > qemu-iotests 122 fails for me with this patch.
> > > I will have a look, where and why exactly it fails, but the allocation
> > > pattern might be slightly different due to the alignment. What counts
> > > is that the output is byte identical or not?
> > Right, I noticed only after sending this email that it's qemu-img map
> > output that changes and this might actually be okay. I didn't check,
> > however, if the exact changes are what is expected and whether we need
> > to add more test cases to cover what the test originally wanted to
> > cover.
> > 
> > So after all, there's a good chance that all that's missing is just an
> > update to the test case.
> > 
> > Kevin
> 
> I checked the output of test 122 and what happens is exactly what is expected.
> The zero areas align to 4k or 8k respectively. If they don't align they are reported
> as allocated. I would just go ahead and include the test update and send V6 if there
> are no objections.

Sounds good to me.

Kevin

> If someone feels that the behaviour is undesired we can also just go ahead
> with a light version of the patch and use only bs->request_alignment as target
> alignment and ignore the value of min_sparse.
> 
> In this case we could even think about as treating this as a bug fix as it
> avoids these ugly RMW cycles that we were seeing and this is why we created this
> patch.
> 
> Best,
> Peter
> 
> -- 
> 
> Mit freundlichen Grüßen
> 
> Peter Lieven
> 
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