Am 07.09.2010 17:30, schrieb Anthony Liguori: > On 09/07/2010 10:20 AM, Kevin Wolf wrote: >> Am 07.09.2010 17:11, schrieb Anthony Liguori: >>> Copy-on-read is, in many cases, a property of the backing file because >>> it suggests that the backing file is either very slow or potentially >>> volatile. >>> >> The simple copy-on-read without actively streaming the rest of the image >> is not enough anyway for volatile backing files. >> > > But as a web site owner, it's extremely useful for me to associate > copy-on-read with an image because it significantly reduces my bandwidth. > > I have a hard time believing this isn't a valuable use-case and not one > that's actually pretty common. As a web site user, I don't necessarily want you to control the behaviour of my qemu. :-) But I do see your point there. >>> You really can't do as good of a job in the block layer because you have >>> very little info about the characteristics of the disk image. >>> >> I'm not saying that the generic block layer should implement >> copy-on-read. I just think that it should pass a run-time option to the >> driver - maybe just a BDRV_O_COPY_ON_READ flag - instead of having the >> information in the image file. From a user perspective it should look >> the same for qed, qcow2 and whatever else (like copy-on-write today) >> > > Okay, the only place I'm disagreeing slightly is that I think an image > format should be able to request copy_on_read such that the default > behavior if an explicit flag isn't specified is to do what the image > suggests we do. Maybe we can agree on that. I'm not completely decided yet if allowing the image to contain such a hint is a good or a bad thing. Kevin -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list