Hans Reiser wrote on Tue, 31 May 2005 11:32:04 -0700: > What about if we have it that only the first name a directory is created > with counts towards its reference count, and that if the directory is > moved if it is moved from its first name, the new name becomes the one > that counts towards the reference count? A bit of a hack, but would work.
Sounds a lot like what I did earlier. Files got really deleted when the true name was the only name for a file (only one parent in other words). But I also had a large cycle finding pause when any file movement happened. I'm not sure if it would still be needed. Nikita Danilov wrote: > - if garbage collection is implemented through the reference counting > (which is the only known way tractable for a file system), then cycles > are never collected. > [...] > But the garbage collection problem is still there. You are more than > welcome to solve it by implementing generation mark-and-sweep GC on file > system scale. :-) There are at least two choices: Bite the bullet and have a file system that is occasionally slow due to cycle checking, but only when the user somehow makes a huge cycle. Keep in mind that this only happens when you use the new functionality, if you only create files with one parent, it should be as fast as regular file systems. I see its features being useful for desktop use, not servers, so the occasional speed hit is less annoyance than the lack of features (the ability to file your files in several places). Another way is to not delete the files when they get unlinked. Similar to some other allocation management systems, have a background thread doing the garbage collection and cycle tracing. The drawback is that you might run out of disc space if you're creating files faster than the collector is cleaning up. I wonder if you can combine a wandering journal (or whatever it is called, where the journalled data blocks become the file's current contents) with the copy type garbage collection (is that the same as a 2 generation mark and sweep?). Copy type collection copies all known reachable objects to an empty half of the disk. When that's done, the original half is marked empty and the next pass copies in the other direction. Could work nicely if you have two disk drives. Yet another PhD topic on garbage collection for someone to research :-) There are lots of other garbage collection schemes that might be applicable to file systems with cycles. It could work, maybe with decent speed too! - Alex
