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 <kw...@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 <p...@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 > > ........................................................... > > KAMP Netzwerkdienste GmbH > Vestische Str. 89-91 | 46117 Oberhausen > Tel: +49 (0) 208.89 402-50 | Fax: +49 (0) 208.89 402-40 > p...@kamp.de | http://www.kamp.de > > Geschäftsführer: Heiner Lante | Michael Lante > Amtsgericht Duisburg | HRB Nr. 12154 > USt-Id-Nr.: DE 120607556 > > ........................................................... > >