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
