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Re: FSFS instance-id and on-disk representation

From: Evgeny Kotkov <evgeny.kotkov_at_visualsvn.com>
Date: Fri, 17 Feb 2017 00:40:31 +0300

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
Received on 2017-02-16 22:41:03 CET

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