(This post is still on-topic, but hanging by a thread.)
Magnus Bodin <[EMAIL PROTECTED]> wrote:
> On Thu, Feb 10, 2000 at 11:21:08PM -0500, Len Budney wrote:
> > ...the reliability impact of soft updates on FFS--giving us a good
> > example of both a reliable journalled filesystem, and an unreliable
> > journalled filesystem.
>
> Thanks. Yes. Let's make the clarification. FFS with soft updates is NOT a
> journaling file system.
Thanks for the correction. Do you have a reference for that? I couldn't
find one.
It is possible to use soft update technology while still maintaining a
journal of metadata. I assumed, apparently incorrectly, that FFS did
this. (If the metadata journal has been discarded, I would have thought
that the filesystem would be different enough to warrant a new name.)
> It DOES NOT MEAN that rename() is no longer an atomic operation in respect
> to other applications, but if you get a crash between the rename() and the
> fsync() you cannot be sure that the rename has been done when you come up
> again.
>
> The impact of qmail is however dim to me.
qmail-queue never calls rename; only link. If I'm wrong in assuming
that your remarks transfer to link, please let me know. With that
disclaimer, here's how qmail-queue enqueues a message:
1. Create a file in a temporary directory (queue/pid). It's inode
becomes the message number (say 457).
3. Link the tempfile to queue/mess/457.
4. Write the message to queue/mess/457 (via the open file descriptor
in queue/pid). fsync().
5. Create queue/intd/457, and write the envelope to the file. fsync().
6. link queue/intd/457 to queue/todo/457.
If the link in #6 succeeds, qmail-queue will return success. If the link
in queue/todo later vanishes, then so does the mail message.
> Do we have to add extra fsyncs here?
I'm not sure. In your other post (subject: ``fsync semantics''), you
didn't supply the arguments to fsync.
Is it necessary to open each of queue/mess, queue/intd and queue/todo
and fsync them? In that case we need three more fsyncs (under FFS with
softupdates, at least).
Does fsyncing an open file descriptor sync not only metadata but hard
links as well? In that case fsyncing queue/intd/messnum again after
#6 suffices.
Len.
--
In general, I'm not going to blindly copy sendmail features---even
useful features. I want to understand what problem they're trying to
solve; then I'll provide the best solution for that problem.
-- Dan Bernstein, author of qmail
