On Thu, 2020-07-02 at 21:37 +0300, Konstantin Kharlamov wrote: > On Thu, 2020-07-02 at 09:44 +0200, Florian Weimer wrote: > > * Konstantin Kharlamov: > > > > > FWIW, I was just thinking about it, and I came up with example you > > > may like which shows exactly why BTRFS is bad for HDD. Consider > > > development process. It includes rewriting source files over and > > > over: you do `git checkout foo` and files are overwritten, you > > > change a file in text editor, and it gets overwritten. And since > > > BTRFS is CoW, it will always write files to a new place. > > > > Editors that make a backup copy typically do not overwrite files in > > place. They rename the file to the backup location and then write the > > new file. > > > > git checkout unlinks changed files first, before writing them anew > > from scratch. > > > > A COW file system does not make a difference for these use cases > > because there is already COW at the application level. > > > > The GNU assembler truncates the output object file first. On XFS, > > that triggers relocation to a new file system location as well, even > > if the output file size (or contents) does not change. So that > > scenario is essentially COW as well today. > > Per my understanding what happens when you write a new file and delete an old > one is that a block that old file was taking gets freed. > > Then, if you copy the file again, file system should find a free block to > write this copy into. And this block likely would be the one that got freed > previously. > > So, well, it is indeed COW, but not the one BTRFS does. It's a COW that copies > a file back and forth between two blocks :) This is kinda HDD-friendly COW :) > > BTRFS on the other hand will not rewrite older block unless it's out of new > ones. Just to clarify: I do not claim this is how ext4 or xfs works. This simplistic explanation is just something obvious regarding how a non-COW fs would work, but of course there can be reasons for them to behave differently. If someone knows better, they're welcome. What I do know though, is how a COW FS works, because I did work a little with ZFS at dayjob. _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
