Re: UBIFS: Do we need to fsync the parent directory?

[Richard Weinberger <richard.weinberger@xxxxxxxxx>]

On Mon, Dec 10, 2018 at 1:16 PM Guillermo Rodriguez Garcia
<guille.rodriguez@xxxxxxxxx> wrote:
>
> Hi all,
>
> When using the typical and well-known approach for atomic file updates:
>
> fd = open(“foo.new”, O_WRONLY);
> write(fd, buf, bufsize);
> fsync(fd);
> close(fd);
> rename(“foo.new”, “foo”);
>
> And assuming that we want to ensure not only that the update was
> atomic, but also that the "new" version of the file will be available
> in the event of a crash, normally one would also fsync the parent
> directory.
>
> Is this needed in UBIFS? The reason I am asking is this snippet from
> the online documentation:
>
> > fsync() may be called for directories - it synchronizes the directory inode meta-data. [...]
> > The fdatasync() call for directories is "no-op" in UBIFS and all UBIFS operations which change directory entries are synchronous.
>
> If I am reading correctly, the latter ("all UBIFS operations which
> change directory entries are synchronous") seems to imply that calling
> fsync on the parent dir would not be necessary. However my tests seem
> to indicate otherwise. Am I misreading the docs?

I fear the docs are ambiguous.
You need to sync the parent too, like on any other filesystem.

-- 
Thanks,
//richard

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