While the design of ext3 in the regard to this bug might be considered
accidental, it would be wise to attempt to carry it over to ext4 in
order to go 'above and beyond' POSIX in compatibility with previous
behaviour. Specifically, truncation of a file needs to be made a regular
write operation so that it would be cached with other write operations
and flushed to disk in a regular batch.
Given the following code:
1.a) open and read file ~/.kde/foo/bar/baz
1.b) fd = open("~/.kde/foo/bar/baz", O_WRONLY|O_TRUNC|O_CREAT) --- this
truncates the file
1.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
1.d) close(fd)
Assuming that less than 30 seconds pass between 1.b and 1.c, these two
operations must be executed at the same write cycle without allowing a
significant window of opportunity for major data loss.
2.a) open and read file ~/.kde/foo/bar/baz
2.b) fd = open("~/.kde/foo/bar/baz.new", O_WRONLY|O_TRUNC|O_CREAT)
2.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
2.d) close(fd)
2.e) rename("~/.kde/foo/bar/baz.new", "~/.kde/foo/bar/baz")
It is even clearer here - why would the rename operation change the
destination file before the previous operations are completed? It should
not - the rename must be an atomic operation, even if POSIX does not
demand it. This is an expected behaviour for extN filesystem and ext4
needs to document and honor that.
I understand that a program can not be certain that data will reach the
disk unless some sort of fsync() is called. But destroying old data and
then delaying writing the new version _is_ an ext4 bug, regardless of
what POSIX says.
And as a sidenote - maybe programmers feel differently, but system
administrators much prefer to have a bunch of small text files that we
can edit with text editors and all kinds of scripts instead of SQL
database stores for application configuration. Configuration registry is
a cool principle, but a horrible practice even in bast implementations.
--
Ext4 data loss
https://bugs.launchpad.net/bugs/317781
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