On Wednesday 20 October 2004 17:53, Benjamin Marzinski wrote: > I think I am having a terminology issue here. > > If you do a dmsetup to create a csnap device with (for example) > /dev/sda1 as the origin and /dev/sda2 as the snapstore, and do > another dmsetup to create a csnap device with /dev/sdb1 as the origin > and /dev/sdb2 as the snapstore, then you have two.... what? > > csnap devices? Two virtual csnap devices. > physical snapshot+origin devics in the device mapper stack? Two physical snapshot+origin pairs. > For the sake of future discussions, it would be really nice to have > concrete terms to the things that the dmsetup gives you (possibly > csnap devices) and the things the csnap-create gives you (possibly > virtual snapshot devices). > > If you have concrete names for these > things, tell me, and I'll use them. If not, pick some, so that we > agree. The thing dmsetup gives you is a "virtual device" because device mapper invents it using the underlying "physical" devices (which may themselves be virtual devices, just to confuse things). I say "virtual csnap device" when I want to be unambiguous, and we have both "origin virtual csnap device" and 64 possible flavors of "snapshot virtual csnap device". The thing csnap-create creates by writing onto a "physical" device may deserve a name, something like meta-device, because it supports N distinct virtual devices on top of it. Currently, I just call it a snapshot store, though I've sometimes thought about inventing the term "metadevice". I refer to the underlying devices as "physical" devices (complete with quotes), and we can unambiguously talk about the snapshot store (better than talking about the "snapshot origin" because that gets confused with the origin virtual device, and eventually we will allow snapshot stores with no origins). > Otherwise, as long as we are ignoring the case where you have > multiple csnap devices (a term I use as suggested in the last > paragraph), and hence, multiple agents, the idea makes sense. I'd hope it makes sense even if we don't ignore the multiple snapshot stores case. It comes down to a question of the addressing scheme for our cluster control messages, do we implement the scheme in the agents, or in cman? We could for instance have cman hand out a port for the agent to listen on along with instantiating a service group, giving us a way to message by node:service (analogous to address:port). I just think it's a good idea to try out the simple case first, then have the benefit of experience in settling on a nice addressing scheme. As you've noticed, it's easy to get tangled up in terminology, and the number of different flavors of "multiple" we've got is already intimidating. One thing we don't want to do is require all the servers for multiple snapshot stores to be on the same node, so far there is no such requirement. Regards, Daniel
