Re: [libvirt] [RFC v2] external (pull) backup API

>> 1. It looks unsafe to use nbd server + backup(sync=none) on same node, >> synchronization is needed, like in block/replication, which uses >> backup_wait_for_overlapping_requests, backup_cow_request_begin, >> backup_cow_request_end. We have a filter driver for this thing, not yet >> in upstream. > Is it the case that blockdev-backup sync=none can race with read > requests on the NBD server? > > i.e. we can get temporarily inconsistent data before the COW completes? > Can you elaborate? I'm not sure but looks possible: 1. start NBD read, find that there is a hole in temporary image, decide to read from active image (or even start read) and yield 2. guest writes to the same are (COW happens, but it doesn't help) 3. reduce point (1.), read invalid (already updated by 2.) data And similar place in block/replication, which uses backup(sync=none) too is protected from such situation.