I've enjoyed using Irmin so far -- the APIs are well-designed and it's really handy to be able to use 'git log' for debugging. In fact a 'git log --pretty=online' has started to replace a lot of the ad-hoc debug logging which permeates my code.
Given how cheap storage is now, I think there are a lot of applications for which it makes sense to record all previous states by default (e.g. for debugging) and make mutating history (by squashing/compacting it away) the special case (a bit like running 'logrotate' every week)
Given how cheap storage is now, I think there are a lot of applications for which it makes sense to record all previous states by default (e.g. for debugging) and make mutating history (by squashing/compacting it away) the special case (a bit like running 'logrotate' every week)