[Xapian-discuss] Atomic DB rebuilds

[James Aylett]

On Mon, Oct 04, 2004 at 05:26:34PM +0100, Olly Betts wrote:

> > But if I use symlinks (and I do) in /var/lib/omega/data to point to the 
> > db - well as we know, its impossible to atomically update a symlink, so 
> > I can't update another DB and then symlink to it once done.
> 
> Do we know that?
> 
> You can certainly create a new symlink with a temporary name, then use
> rename() to replace the old symlink with the old one.  And according to
> "man 2 rename":
> 
>     If newpath already exists it will be atomically replaced (subject to a
>     few conditions - see ERRORS below), so that there is no point at which
>     another process attempting to access newpath will find it missing.
> 
> Nothing in the ERRORS section seems to say that symlinks aren't replaced
> atomically.

I wouldn't rely on it. I have a horrible feeling that it won't always
work under NFS ...

Didn't Richard write a psuedo symlink backend that took a single file
to configure it to point to where the actual data was? So you could
rename the _file_ over the old one, which certainly is atomic, and
will work on all NFS implementations that include lockd (at least,
AFAIK).

J

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  James Aylett                                                  xapian.org
  james at tartarus.org                               uncertaintydivision.org