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Subject(fwd) Re: [RFC] mount flag "direct"
Sorry I'm getting behind with the mail. Meetings, and I'm flying
towmorrow.

> On Tue, 3 Sep 2002, Peter T. Breuer wrote:
> > If it doesn't cause the data to be read twice, then it ought to, and
> > I'll fix it (given half a clue as extra pay ..:-)
>
> writing then reading the same file may cause it to be read from the disk,
> but reading /foo/bar then reading /foo/bar again will not cause two reads
> of all data.

Hmm. I just did a quick check on 2.5.31, and to me it looks as though
two consequtive reads BOTH drop through to the driver. Yes, I am
certain - I've repeated the experiment 4 times reading the same 400K,
and each time the block driver registers 400 read requests.

> some filesystems go to a lot fo work to orginize the metadata in
> particular in memory to access things more efficiantly, you will have to
> go into each filesystem and modify them to not do this.

Well, I'll have to divert them. Is there not some trick that can be
used? A bitmap mmapped to the device, if that's not nonsensical in
kernel space?

> in addition you will have lots of potential races as one system reads a
> block of data, modifies it, then writes it while the other system does the

Uh, I am confident that there can be no races with respect to data
writes provided I manage to make the VFS operations atomic via
appropriate shared locking. What one has to get rid of is cached
metadata state. I'm open to suggestions.

> yes this is stuff that could be added to all filesystems, but will the
> filesystem maintainsers let you do this major surgery to their systems?

Depends how a patch looks, I guess.

> for example the XFS and JFS teams are going to a lot of effort to maintain
> their systems to be compatable with other OS's, they probably won't

Yes.

> appriciate all the extra conditionals that you will need to put in to
> do all of this.

I don't see any conditionals. These will be methods, i.e. redirects.
Anyway, what's an if test between friends :-).

> even for ext2 there are people (including linus I believe) that are saying
> that major new features should not be added to ext2, but to a new

I agree! But I wouldn't see adding VFS ops to replace FS-specific
code as a new feature, rather a consolidation of common codes.

> filesystem forked off of ext2 (ext3 for example or a fork of it).

Peter
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