Alexander G. M. Smith wrote: >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). > > I prefer the above to the below.
>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 > > > >
