On Thu, Sep 30, 2021 at 07:22:29AM +0000, Martin Wilck wrote: > On Wed, 2021-09-29 at 23:39 +0200, Peter Rajnoha wrote: > > For event-based activation, I'd expect it to really behave in event- > > based manner, that is, to respond to events as soon as they come and not > > wait for all the other devices unnecessarily. > > I may be missing something here. Perhaps I misunderstood David's > concept. Of course event-based activation is best - in theory. > The reason we're having this discussion is that it may cause thousands > of event handlers being executed in parallel, and that we have seen > cases where this was causing the system to stall during boot for > minutes, or even forever. The ideal solution for that would be to > figure out how to avoid the contention, but I thought you and David had > given up on that. > > Heming has shown that the "static" activation didn't suffer from this > problem. So, to my understanding, David was seeking for a way to > reconcile these two concepts, by starting out statically and switching > to event-based activation when we can without the risk of stalling. To > do that, we must figure out when to switch, and (like it or not) udev > settle is the best indicator we have. > > Also IMO David was striving for a solution that "just works" > efficiently both an small and big systems, without the admin having to > adjust configuration files. Right, this is not entirely event based any longer, so there could be some advantage of an event-based system that we sacrifice. I think that will be a good tradeoff for the large majority of cases, and will make a good default. > > The use of udev-settle is always a pain - for example, if there's a mount > > point defined on top of an LV, with udev-settle as dependency, we practically > > wait for all devices to settle. With 'all', I mean even devices which are not > > block devices and which are not event related to any of that LVM > > layout and the stack underneath. So simply we could be waiting uselessly and we > > could increase possibility of a timeout (...for the mount point etc.). One theoretical advantage of an event-based system is that it reacts immediately, so you get faster results. In practice it's often anything but immediate, largely because of extra work and moving parts in the event-based scheme, processing each event individually. So, the simpler non-event-based method will often be faster I think, and more robust (all the moving parts are where things break, so best to minimize them.) You've filled in some interesting details about udev-settle for me, and it sounds like there are some ideas forming about an alternative, which would offer us a better way to switch to event-base-mode. I'd like to be able to simply replace the systemd-udev-settle dependency with an improved "new-settle" dependency when that's ready. Dave _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
