On Tue, Jun 08, 2021 at 08:26:01AM +0000, Martin Wilck wrote: > IIUC, 2) is the effect of _pvscan_aa_quick(). 3) is surprising; > apparently libudev's device detection causes a factor 3 slowdown. > While 40s is not bad, you can see that event based activation still > performs far worse than "serial" device detection lvm2-activation- > early.service. > > Personally, I'm sort of wary about obtain_device_list_from_udev=0 > because I'm uncertain whether it might break multipath/MD detection. > Perhaps you can clarify that. Yes, that's an issue, but it's something we've needed to clean up for a while, and I made a small start on it a while back. obtain_device_list_from_udev is supposed to only control whether lvm gets a list of device names from readdir /dev, or from libudev. My preference is default 0, readdir /dev. This avoids the performance problem and makes lvm less dependent on the vagaries of udev in general. But, as you and Peter mentioned, obtain_device_list_from_udev also became entangled with md/mpath detection methods, which is more closely related to external_device_info_source=udev|none. I think it would be an improvement to: . Make obtain_device_list_from_udev only control how we get the device list. Then we can easily default to 0 and readdir /dev if it's better. . Use both native md/mpath detection *and* udev info when it's readily available (don't wait for it), instead of limiting ourselves to one source of info. If either source indicates an md/mpath component, then we consider it true. The second point means we are free to change obtain_device_list_from_udev as we wish, without affecting md/mpath detection. It may also improve md/mpath detection overall. A third related improvement that could follow is to add stronger native mpath detection, in which lvm uses uses /etc/multipath/wwids, directly or through a multipath library, to identify mpath components. This would supplement the existing sysfs and udev sources, and address the difficult case where the mpath device is not yet set up. 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/
