I created a small program to help me synchronize files in sort-of real-time between two directories (the idea is that one of them is on a NFS server). There are no replicated file systems for FreeBSD and the canonical way to do this is usually to use rsync or something like it. The problem here is that rsync always traverses both directory structures, compares files and then copies them (via a variety of smart algorithms but it’s still very resource-intensive).
So I created a daemon that uses kqueue(2) to monitor which files changed and feed only those files to rsync (it’s not exactly a new idea, I’m sure somebody has also mentioned it somewhere on the FreeBSD lists). This is in many ways a suboptimal solution since it needs to keep an open file descriptor for all the monitored files (which ties in kernel resources and memory) so it won’t scale for really large directories, which could actually benefit the most from this approach. It will work reasonably well for a small number of files (up to several tens of thousands), with modifying kern.maxfiles and kern.maxfilesperproc sysctls and login session limits (if applicable).
Anyone who’s interested can download the adfs daemon and try it. This was hacked together over the weekend so it probably has some problems. I’ll fix those problems that prevent me from using it, but I’ll update the online archive only if there are interested users.