this post was submitted on 14 Aug 2023
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Copy on write is likely to introduce significant performance decreases in cases where large or medium size files have a couple bytes changed. It’s usually recommended to turn CoW off on those files; I found it to be more hassle than it’s worth for a root filesystem. It is still a reasonable file system for file storage that looks more like archival - files land there and seldomly or never change. If you don’t have a specific need in mind though, I wouldn’t bother - in my opinion, it’s not great as a general purpose filesystem.
Do you happen to have a source or benchmark for this? My understanding of CoW is that the size of the file does not matter, as BTRFS works with blocks and not files. When a block is changed, it's written to a new location. All the old blocks that are not changed are not written again - this wouldn't even make sense in the context of how BTRFS deduplicates blocks anyway.
So:
10 kB base file
modify 1kB of the content
== 11kB total "used" space, and 1kB of new written blocks.
that old 1kB that is no longer part of the file will eventually be cleaned up if needed, but there's no reason to delete it early.
Why would OpenSuse/Fedora choose it as their default filesystem if it was as bad you describe?
Thanks for the advice!
How do you define medium or large files? What examples of these exist on root?