Am 19.07.2013 um 08:29 hat Wenchao Xia geschrieben: > δΊ 2013-7-17 22:23, Eric Blake ει: > >On 07/17/2013 08:03 AM, Wenchao Xia wrote: > >>This series allow user to read internal snapshot's contents without qemu-img > >>convert. Another purpose is that, when qemu is online and have taken an > >>internal snapshot, let user invoke qemu-nbd to do any thing on it except > >>write. > >> > >>This brings two interesting issues: > >>1 is it safe to let qemu-nbd and qemu access that file at same time? > > > >Probably not, for the same reason we tell people to not use qemu-img > >while qemu is active on a file. > > > For external case, or backing chain, qemu-nbd export while qemu is > active, do you think it is OK? > > base->imageA > > qemu-nbd export base > qemu use imageA.
It depends. If you use 'qemu-nbd -r' to export it, it's okay. The reason is that it's allowed to have either one process opening the file read-write, or an arbitrary number of processes opening the file read-only. In the case of qemu using imageA, base is only opened read-only, so qemu-nbd can be another read-only user. Kevin
