On the 28.12.2010 23:54, Olaf van der Spek wrote:
On Tue, Dec 28, 2010 at 11:31 PM, Neil Brown<neilb@xxxxxxx> wrote:
True, but all those exceptions (IMO) should be (proven to be) no problem.
I'd prefer designs that don't have such exceptions. I may not be able
to think of a concrete problem right now, but that doesn't mean such
problems don't exist.
Very true. But until such problems are described an understood, there is not
a lot of point trying to implement a solution. Premature implementation,
like premature optimisation, is unlikely to be fruitful. I know this from
experience.
The problems seem clear. The implications not yet.
I also don't understand why providing this feature is such a
(performance) problem.
Surely the people that claim this should be able to explain why.
Without a concrete design, it is hard to assess the performance impact. I
would guess that those who anticipate a significant performance impact are
assuming a more feature-full implementation than you are, and they are
probably doing that because they feel that you need the extra features to
meet the actual needs (and so suggest those needs a best met by a DBMS rather
than a file-system).
Of course this is just guess work. With concreted reference points it is
hard to be sure.
True, I don't understand why people say it will cause a performance
hit but then don't want to tell why.
We are talking about atomicity. And it is a simple fact in the field of
information processing/informatics/computer science that if someone
wants to give/have the guarantee of atomicity, then she/he has to do
several additional steps often by using an additional data structure. In
the end this all costs more time and/or space than doing it without
atomicity. At this point there is no discussion anymore, because this is
fully discussed to the maximum in subjects like Efficient Algorithms,
Special Problem Fields of Operating System Design and Fundamentals of
DBMS Design (eg. AtomicityCID principle).
And such fundamental points are not (needed to be) discussed here.
Furthermore, due to the competence it is possible for FS gurus like Ted
to estimate that the additional steps have to be done by several
functions of an FS, which implies performance loss. And because
elementary FS functions are involved the performance loss could be and
in the past have been significant, though in nearly all cases I have
seen the reason was a very bad implementation. The only exception so far
is the Reiser4 FS: All of its file operations are atomic, but still to a
little cost of performance in the most cases and the need of a repacker
in some few cases which show a significant loss of performance.
And the advice to use a well-known DBMS is simply based on the knowledge
that it has all the needed functionality already implemented in a highly
performant way, and on the knowledge that such a solution is used
oftenly for comparable use cases due to the cost vs. benefit ratio.
To take a look at the Reiser4 FS could also help.
<snip>
Olaf
The bar is opened
Christian Stroetmann
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