Re: FSFS instance-id and on-disk representation

Evgeny:

Thank you.

Doug

On Thu, Feb 16, 2017 at 4:40 PM, Evgeny Kotkov <evgeny.kotkov_at_visualsvn.com>
wrote:

> Julian Foad <julianfoad_at_apache.org> writes:
>
> > == Question ==
> >
> > WANdisco would like to know that there will not be differences in the
> > repository on-disk data due to differences in instance-id (other than the
> > "db/uuid" itself, of course). I suggest we are talking about the
> lifetime of
> > FSFS format 7; of course the features of a future format are unknown.
> >
> > Can I tell them that that is the expectation, and we won't change that
> > situation without a good reason?
>
> The instance ID was added to handle a case when two repositories with
> the same UUID (say, one was hotcopied or dump/loaded from another) are
> opened within a single process.
>
> Without the instance ID, these repositories share internal data that should
> not be shared, such as the transaction list and mutexes. This can result
> in various types of errors or deadlocks. An instance ID makes it possible
> to distinguish the internal data for such near-duplicate repositories, and
> is not used anywhere else.
>
> Answering the question, it is safe to assume that an instance ID doesn't
> change what gets written to the disk (apart from the 'db/uuid' contents),
> and that this will not change in format 7.
>
> Hope this helps :)
>
>
> Regards,
> Evgeny Kotkov
>

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