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SubjectRe: [RFC] mount flag "direct"
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"A month of sundays ago Lars Marowsky-Bree wrote:"
> "Peter T. Breuer" <ptb@it.uc3m.es> said:
>
> > No! I do not want /A/ fs, but /any/ fs, and I want to add the vfs
> > support necessary :-).
> >
> > That's really what my question is driving at. I see that I need to
> > make VFS ops communicate "tag requests" to the block layer, in
> > order to implement locking. Now you and Rik have pointed out one
> > operation that needs locking. My next question is obviously: can you
> > point me more or less precisely at this operation in the VFS layer?
> > I've only started studying it and I am relatively unfamiliar with it.
>
> Your approach is not feasible.

But you have to be specific about why not. I've responded to the
particular objections so far.

> Distributed filesystems have a lot of subtle pitfalls - locking, cache

Yes, thanks, I know.

> coherency, journal replay to name a few - which you can hardly solve at the

My simple suggestion is not to cache. I am of the opinion that in
principle that solves all coherency problems, since there would be no
stored state that needs to "cohere". The question is how to identify
and remove the state that is currently cached.

As to journal replay, there will be no journalling - if it breaks it
breaks and somebody (fsck) can go fix it. I don't want to get anywhere
near complicated.

> VFS layer.
>
> Good reading would be any sort of entry literature on clustering, I would

Please don't condescend! I am honestly not in need of education :-).

> recommend "In search of clusters" and many of the whitepapers Google will turn
> up for you, as well as the OpenGFS source.

(Puhleeese!)

We already know that we can have a perfectly fine and arbitrary
shared file system, shared only at the block level if we

1) permit no new dirs or files to be made (disable O_CREAT or something
like)
2) do all r/w on files with O_DIRECT
3) do file extensions via a new generic VFS "reserve" operation
4) have shared mutexes on all vfs op, implemented by passing
down a special "tag" request to the block layer.
5) maintain read+write order at the shared resource.

I have already implemented 2,4,5.

The question is how to extend the range of useful operations. For the
moment I would be happy simply to go ahead and implement 1) and 3),
while taking serious strong advice on what to do about directories.



Peter
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