Messages in this thread | | | Date | Tue, 3 Sep 2002 23:48:32 +0200 | From | "Peter T. Breuer" <> | Subject | (fwd) Re: [RFC] mount flag "direct" |
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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).
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