On 10/05/2016 02:40 AM, Alasdair G Kergon wrote: > On Tue, Oct 04, 2016 at 04:39:28PM -0700, Andy Grover wrote: >> devicemapper is using uevents for: >> a. dm-verity detected corruption >> b. dm-multipath: path failed or reinstated >> c. dm device renamed >> d. there's also some use in md and bcache. >> >> devicemapper uses DM_EVENT ioctl (yuck) for: >> 1. dm-thin pool data/metadata filling up (hit a threshold) >> 2. dm-cache is now clean >> 3. dm-log flushed or log failed >> 4. dm-raid error detected or sync complete > >> there doesn't seem to be much technical differentiation between the >> two lists. > > The distinction in dm is that events in the first category may affect > the availability of the device: they represent major (and hopefully > rare) changes. > > Events in the second category are just notifications: no impact on /dev, > no need to trigger udev rules, and their use will continue to be > extended, and (rarely at the moment) could be frequent (which is no > problem for the existing polling-based mechanism). > >> Instead of using uevent for everything, we could go to a separate >> genetlink for 1-4 instead of making them use uevent like a-d, but we'd >> end up with two different userspace notification techniques. > > We see these as two different categories of notifications, and prefer > the greater flexibility a mechanism independent of uevents would > provide. The team has discussed several alternatives over the years but > didn't make a decision as we've not yet reached a point where we're > straining the existing mechanism too far. > I would love to remove the dm-event usage from multipathing. As it stands using dm-events from multipathing is a massive resource drain (we have to allocate a waiting thread for each device), and we more-or-less disregard that information anyway as it is an ex-post notification made _after_ someone modified the table. Given that in most cases it was actually _us_ doing the reconfiguration it has very little value. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
