[OpenAFS-devel] vos dump busying out the volume while dumping...

[Hartmut Reuter]

Michael Niksch wrote:

> So would it be possible to do perform 'vos backup' and 'vos
> addsite,release' on such a differently named clone and end up with 5
> clones of the same clone (plus any dynamic clones during 'vos move' or
> 'vos copy'). Would it be possible to perform multiple 'vos copy' and
> end up with an essentially unlimited number of clones? I guess there
> must be something like a reference count for a vnode that can overflow,
> at which point it will look as if it were zero and make your data
> disappear altogether. At which number of clones would that happen - 8,
> 256, 64K ?

In the NAMEI-implementation the number of different versions of a file 
and the number of references to a single version of a file are limited 
by design because this information must take place in a volume special 
file called linktable. Per vnode 16 bits are used which subdivide in 5 
portions of 3 bits each containing the reference count of a single 
version. That means a single version of a file can be referenced not 
more than 7 times (which fits in 3 bits).

src/vol/namei_ops.c

#define NAMEI_TAGMASK      0x7

#define NAMEI_MAXVOLS 5 /* Maximum supported number of volumes per volume
                       * group, not counting temporary (move) volumes.
                       * This is the number of separate files, all having
                       * the same vnode number, which can occur in a volume
                       * group at once.
                       */

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Hartmut Reuter                           e-mail reuter@rzg.mpg.de
					   phone +49-89-3299-1328
RZG (Rechenzentrum Garching)               fax   +49-89-3299-1301
Computing Center of the Max-Planck-Gesellschaft (MPG) and the
Institut fuer Plasmaphysik (IPP)
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