Re: ext3-2.4-0.9.4

Jan Harkes
2001-08-03T15:18:19Z

On Fri, Aug 03, 2001 at 04:08:20PM +0200, Daniel Phillips wrote:
> > fsync forces the data to disk. There may be one or more pathnames
> > which the application also relies on, and if the application does
> > care about those, the application will have to ensure that they are
> > synced too.
> >
> > Look, I agree that your interpretation is useful. It's just not an
> > unambiguous requirement of the spec.
>
> OK, fine, this damn English language and all that. Do we agree that
> "access" means "be able to open by name"?

No, until recently the device/inode number pair used to work very nicely
for both Coda and knfsd when they wanted to access a file. But it only
works from within the kernel where you can use iget. It's just that with
some of the newer filesystems the inode numbers are no longer unique, so
it became something more like device/inum/opaque handle (i.e. iget4).

As far as the fsync semantics are concerned. Yeah, they would be useful,
but only to avoid one directory fsync call that will tell the kernel
_exactly_ what the process wants instead of letting it figure it out by
itself. The argument I saw in this thread that fsync(dir) has too much
overhead because it might sync unrelated data is not very useful,
because that unrelated data will be synced anyways when it's not a
journalling fs.

Name to file object mapping is not part of the metadata associated with
a file. It is the contents of the directory and can only be modified by
directory operations, not operations on the file or filehandle.
open(file, O_CREAT) is really split up into create(parent, file) an
operation on the parent directory, and an open(file) operation which
returns the filehandle.

I also don't see why a rename operation, which operates on the source
and destination parent directories would have to not only look up the
file object but also somehow register with all open filehandles for that
object that both olddir and newdir need to be written back to disk
during the fsync as well. Or should that go the other way around, where
the filehandle has to chase down which directory it was renamed to?

Taken from another part of this thread Alexander and Daniel,
> > That there isn't THE directory in which the file resides. There might
> > be several, and setting the bit on one of them at random and expect
> > it to work is a _lot_ of work. For no real use.
>
> There is only one chain of directories from the fd's dentry up to the
> root, that's the one to sync.

Using the dentry chain is not reliable, for instance instead of moving
dentries around Coda simply unhashes dentries when state on the server
changes. Another process (perhaps on a different machine) might have
moved one of the ancestor directories from one location to another, or
even relinked/unlinked the file that we're working with (ln old new; rm
old) in which case the dentry path is lost, but ideally you'd expect
fsync to sync the 'new' name if it supposedly keeps the namespace
consistent.

Working on a distributed filesystem with somewhat weaker than UNIX
semantics might have skewed my vision. In Coda not every client will be
able to figure out which are all of the possibly paths that can lead to
a file object. And although we currently try our best to block
hardlinked directories they could possibly exist, making the problems
even worse.

Jan