5 ms·

I believe you're wrong here. If you modify a hardlinked file, the FS will create new inodes under the hood, leaving existing ones intact. So it's like CoW. At

by koiueo 6d ago

I believe you're wrong here. If you modify a hardlinked file, the FS will create new inodes under the hood, leaving existing ones intact. So it's like CoW. At least on ext4. // Didn't double-check, but I've held this belief for over decade now, would be surprised to be proven wrong

dreamcompiler 6d ago
Changing a hardlinked file changes it everywhere, unless your specific editor happens to create a new inode. Thus it depends on your editor. CoW is similar except the new inode is created by the file system itself and is independent of the editor.
epcoa 6d ago
No this is completely incorrect. The filesystem doesn’t break a link just if you open it. Your editor or higher level utility is free to do so (by unlinking and copying the data into a new file/inode), but this is not how the filesystem works.
drdexebtjl 6d ago
You're about to be very surprised! echo old > a ln a b echo new > a cat a b # prints "new" and "new" Some programs like GNU sed [1] do create new inodes, but that is a property of the application, not the filesystem: echo old > a ln a b sed -i 'c new' a cat a b # prints "new" and "old" With reflinks in a CoW filesystem the inodes are different from the start, but share the same underlying blocks: echo old > a cp --reflink=always a b ls -i a b # prints different inodes echo new > a cat a b # prints "new" and "old" [1] https://www.gnu.org/software/sed/manual/html_node/Command_002dLine-Options.html#index-_002di https://www.gnu.org/software/sed/manual/html_node/Command_00...
- koiueo 6d ago
  Cool, I love surprises :D Thanks for examples and especially the doc link. I don't normally use hard links, and for some reason was sure that's how they behave. Maybe my belief indeed stemmed from observing software which overwrites opened file with a new copy.

by koiueo 6d ago

dreamcompiler 6d ago

Changing a hardlinked file changes it everywhere, unless your specific editor happens to create a new inode. Thus it depends on your editor. CoW is similar except the new inode is created by the file system itself and is independent of the editor.

epcoa 6d ago

No this is completely incorrect. The filesystem doesn’t break a link just if you open it. Your editor or higher level utility is free to do so (by unlinking and copying the data into a new file/inode), but this is not how the filesystem works.

drdexebtjl 6d ago

You're about to be very surprised! echo old > a ln a b echo new > a cat a b # prints "new" and "new" Some programs like GNU sed [1] do create new inodes, but that is a property of the application, not the filesystem: echo old > a ln a b sed -i 'c new' a cat a b # prints "new" and "old" With reflinks in a CoW filesystem the inodes are different from the start, but share the same underlying blocks: echo old > a cp --reflink=always a b ls -i a b # prints different inodes echo new > a cat a b # prints "new" and "old" [1] https://www.gnu.org/software/sed/manual/html_node/Command_002dLine-Options.html#index-_002di https://www.gnu.org/software/sed/manual/html_node/Command_00...

koiueo 6d ago
Cool, I love surprises :D Thanks for examples and especially the doc link. I don't normally use hard links, and for some reason was sure that's how they behave. Maybe my belief indeed stemmed from observing software which overwrites opened file with a new copy.