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MIT's New File System Won't Lose Data During Crashes
jan_jes sends news that MIT researchers will soon present a file system they say is mathematically guaranteed not to lose data during a crash. While building it, they wrote and rewrote the file system over and over, finding that the majority of their development time was spent defining the system components and the relationships between them. "With all these logics and proofs, there are so many ways to write them down, and each one of them has subtle implications down the line that we didn’t really understand." The file system is slow compared to other modern examples, but the researchers say their formal verification can also work with faster designs. Associate professor Nickolai Zeldovich said, "Making sure that the file system can recover from a crash at any point is tricky because there are so many different places that you could crash. You literally have to consider every instruction or every disk operation and think, ‘Well, what if I crash now? What now? What now?’ And so empirically, people have found lots of bugs in file systems that have to do with crash recovery, and they keep finding them, even in very well tested file systems, because it’s just so hard to do.”
Well, ex-Googler Kent Overstreet recently announced a COW filesystem on lkml:
https://lkml.org/lkml/2015/8/21/22
Not ready for production I would say, but looks interesting.
The feature I miss the most is allowing traversal through a directory hierarchy a user has no explicit permissions for to get to a folder they do have permissions for. I find the workarounds for this in other filesystems to be extremely ugly.
In POSIX, that's represented in a directory's mode bits with octal digit 1 (4: list files, 2: create or delete files, 1: traverse). What do you find ugly about mode 751 (owner create or delete, owner and group list, world traverse)?
I also kind of wish TCP/IP had used the network:mac numbering scheme that IPX used.
It does now. An IPv6 address is divided into a 48- or 56-bit network, a 16- or 8-bit subnet, and a 64-bit machine identifier commonly derived from the MAC.