On Wed, Mar 16, 2022 at 04:07:21PM -0500, Eric Blake wrote: > On Tue, Mar 15, 2022 at 01:14:41PM +0000, Richard W.M. Jones wrote: > > The patches seem OK to me, but I don't really know enough about the > > internals of qemu-nbd to give a line-by-line review. I did however > > build and test qemu-nbd with the patches: > > > > $ ./build/qemu-nbd /var/tmp/test.qcow2 > > $ nbdinfo nbd://localhost > > ... > > can_multi_conn: false > > > > > > $ ./build/qemu-nbd -e 2 /var/tmp/test.qcow2 > > $ nbdinfo nbd://localhost > > ... > > can_multi_conn: false > > > > ^^^ Is this expected? It also happens with -e 0. > > Yes, because qemu-nbd defaults to read-write connections, but to be > conservative, this patch defaults '-m auto' to NOT advertise > multi-conn for read-write; you need to be explicit: > > > > > > > $ ./build/qemu-nbd -e 2 -m on /var/tmp/test.qcow2 > > $ nbdinfo nbd://localhost > > ... > > can_multi_conn: true > > either with '-m on' as you did here, or with > > build/qemu-nbd -r -e 2 /var/tmp/test.qcow2 > > where the '-m auto' default exposes multi-conn for a readonly client.
I hit send prematurely - one more thought I wanted to make clear. For _this_ series, the behavior of '-m auto' depends solely on readonly vs. read-write; that being the most conservative choice of preserving pre-existing qemu-nbd behavior (that is, if you don't use -m, the only change in behavior should be the fact that --help output now lists -m). But in future versions of qemu, we might be able to improve '-m auto' to also take into consideration whether the backing image of a read-write device is known-cache-consistent (such as a local file system), where we can then manage to default to advertising multi-conn for those writable images, while still avoiding the advertisement when exposing other devices such as network-mounted devices where we are less confident on whether there are sufficient cache consistency guarantees. Use of explicit '-m off' or '-m on' gives guaranteed results no matter the qemu version, so it is only explicit (or implied) '-m auto' where we might get smarter defaults over time. Thus testing whether advertising multi-conn makes a difference in other tools like nbdcopy thus requires checking if qemu-nbd is new enough to support -m, and then being explicit that we are opting in to using it. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
