On 2016-06-24 01:20, Chris Murphy wrote:
On Thu, Jun 23, 2016 at 8:07 PM, Zygo Blaxell
<ce3g8...@umail.furryterror.org> wrote:
With simple files changing one character with vi and gedit,
I get completely different logical and physical numbers with each
change, so it's clearly cowing the entire stripe (192KiB in my 3 dev
raid5).
You are COWing the entire file because vi and gedit do truncate followed
by full-file write.
I'm seeing the file inode changes with either a vi or gedit
modification, even when file size is exactly the same, just character
substitute. So as far as VFS and Btrfs are concerned, it's an entirely
different file, so it's like faux-CoW that would have happened on any
file system, not an overwrite.
Yes, at least Vim (which is what most Linux systems use for vi) writes
to a temporary file then does a replace by rename. The idea is that
POSIX implies that this should be atomic (except it's not actually
required by POSIX, and even on some journaled and COW filesystems, it
isn't actually atomic).
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