On 03/26/10 07:21 PM, Karen Tung wrote: > I uploaded the install engine feature highlight slides: > http://hub.opensolaris.org/bin/download/Project+caiman/CUE_docs/install-engine-feature-highlights.pdf > > > During the discussion of these slides, we discussed how stop/pause and > resume would work. > Keith brought up a suggestion on resuming from a snapshot of the data > cache in /tmp, > and I initially though it would be OK to add the feature. > Upon further consideration, I don't think it is a good idea to support > it. I would > like to get your opinion on the issue. > > In summary, I proposed for stop and resume to work as follows in the > engine: > > - After successful execution of each checkpoint, a snapshot of the data > cache will be taken. > - If the install target ZFS dataset is available, the data cache snap > shot will be stored in the ZFS > dataset. > - If the install target ZFS dataset is not available, the data cache > snapshot will be stored in /tmp. > - For resumes without terminating the app, resumes are allowed from > any previously successfully executed checkpoint. > - For application that terminates and resumes upon restarting, resumes > are only allowed from > checkpoints that have data cache snapshot saved in the install target > ZFS dataset. > > See slides 10-17 for more details. > > During the discussion, Keith suggested allowing resume to happen even > if the data cache snapshot is not stored in the ZFS dataset, since the > data cache snapshot > stored in /tmp can be used. I thought it would be OK to support that > also during the meeting. > However, after further consideration, I thought of a couple of reasons > for opposing to > the support. > > 1) If we allow a copy of snapshot to be provided to the engine for > resume, we need to provide an > interface for the user/app to determine which copy of snapshot file > belong to which process. > Arguably, one can guess based on timestamp, and knowledge of the > engine..etc.. > However, all those are implementation specific, and depending on how > things evolve, > they are not "official interfaces" that user/applications can count on. >
I'd argue, though, that you have this same problem with respect to the ZFS snapshots, we've just deliberately ignored it. Or am I missing something about how you're expecting to tag them to record the "ownership"? > 2) The current design will remove all copies of snapshot in /tmp when > the application/engine > terminates. If we want to allow resume from a copy of snapshot in /tmp, > we won't be > cleaning up those snapshots, and over time, we clutter up /tmp with a > lot of snapshot files. > Well, this is likely to be true in the applications as well - DC leaves snapshots around deliberately, but the installers probably won't on successful completion. In any event, I think you'll need to make the cleanup behavior controllable in some way, at the very least for debug purposes. So, if they're going to be around... > Based on this, I don't think it is a good idea to "officially" support > resuming from a data > cache snapshot in /tmp. I can probably leave a backdoor in the code to > enable it > somehow if needed. > > I would like to hear your thoughts on this. > I think I'd consider a little more closely why this would or would not be useful to support, as I'm not sure the issues you've raised are all that unique. For example, I'd think target discovery could be relatively expensive on more complex storage topologies such that it may be convenient to have restart capability post-TD. Dave
