Re: [TUHS] tunefs -m 5%

On Fri, Mar 05, 2021 at 01:50:32AM -0800, John Gilmore wrote: I'll point out that BSD FFS, at least in BSD 4.3, reserves 10% of the file system for reserved blocks. The reasoning gave was the same as with ext2, which is performance suffers when the file system gets really fully. It really depends on the workload, but for the worst case workloads, I've seen performance degradations happen earlier than 90% or 95% full. Defragmentation is a hard problem, and it takes a *long* time, and or chews up a lot of write cycles on flash, persistent memory, etc. This is especially true when the file system is very full, since there is very little "swing space" available to copy data around, especially if one is striving for the perfect state (e.g., a files are contiguous, and the free space is contiguous). Defragmentation utilities was something that was only really popular on Windows systems, and in the last, oh, ten years or so, even in the Windows world defrag is not something you'll see in their disk utilities. Ext4 has a defrag program, but it's really primitive, and it only works by attempting to defrag *files*. It doesn't try to defrag *free space*, which is what you really need if you want to try to keep performance up (and have space so as to keep files defragmented). The main reason why is that no one has sponsored work in this space --- probably because it's way cheaper just to over-provision disk space. If someone wants to try to implement it, I have a rough design about how it might be done for ext4 --- but someone has to volunteer $$$ or their own personal time in order to implement it. And if it's only for yourself, it's probably cheaper just to spend a extra few bucks to buy a bigger disk. I suspect it's for similar reasons that none of the legacy Unix systems have defragmentation implemented either. It's not something which makes business case. But hey, if someone is interested in working on it, I'll give the standard open source answer --- patches gratefully accepted. I'll even give you a rough design as a starting point. (And there's no guarantee it will be a good idea for flash, given that seeks are mostly free for flash, and write cycles are a consideration, and how many people are still using HDD's today; newer operating systems, such as Fuschia, have started designing for a world where they only need to care about file systems for flash.) Cheers, - Ted