Mike Silbersack wrote:
>
> On Tue, 6 Feb 2001, Rik van Riel wrote:
>
> > The system call used to guarantee this is fsync (and friends?);
> > if qmail doesn't use it but makes assumptions that aren't true
> > on any decent OS out there ...
> >
> > regards,
> >
> > Rik
>
> Well, the various qmail programs do seem to fsync (though I'm not sure if
> it's in the right places.) In any case, this link seems to throw some
> light on the situation:
>
> ftp://elektroni.ee.tut.fi/pub/qmail_linux_metadata_message
>
> Now, I have no clue if this is correct or not, but the core of the
> explanation given on that page seems to be:
>
> ---
>
> So what is this all about? qmail relies on the BSD semantics of immediate
> update of directories on the disk when link(), unlink(), open() and
> rename() calls are used. But Linux writes them to the disk asynchronously.
> My library loaded before libc changes those calls to do the corresponding
> directory writes too. Then qmail should be reliable against power outages
> also in Linux.
>
> ---
>
> So, does anyone know if that is a correct assertion to make, and if
> softupdates does indeed break it?
kirk said (but I have not completely checked it) that if you fsync a file,
it will effectively fsync all the way back to the root of the filesystem.
I don't know how true this is, but cerainly the inode is updated before
fsync returns. I cannot tell if any directory entries pointing at that file
that have not yet been sync'd are forced out before it returns....
>
> Mike "Silby" Silbersack
>
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