Messages in this thread | | | Date | Wed, 9 Jan 2008 10:43:21 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH][RFC] fast file mapping for loop |
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On Wed, Jan 09 2008, Christoph Hellwig wrote: > On Wed, Jan 09, 2008 at 09:52:32AM +0100, Jens Axboe wrote: > > - The file block mappings must not change while loop is using the file. > > This means that we have to ensure exclusive access to the file and > > this is the bit that is currently missing in the implementation. It > > would be nice if we could just do this via open(), ideas welcome... > > And the way this is done is simply broken. It means you have to get > rid of things like delayed or unwritten hands beforehand, it'll be > a complete pain for COW or non-block backed filesystems.
COW is not that hard to handle, you just need to be notified of moving blocks. If you view the patch as just a tighter integration between loop and fs, I don't think it's necessarily that broken.
I did consider these cases, and it can be done with the existing approach.
> The right way to do this is to allow direct I/O from kernel sources > where the filesystem is in-charge of submitting the actual I/O after > the pages are handed to it. I think Peter Zijlstra has been looking > into something like that for swap over nfs.
That does sound like a nice approach, but a lot more work. It'll behave differently too, the advantage of what I proposed is that it behaves like a real device.
I'm not asking you to love it (in fact I knew some people would complain about this approach and I understand why), just tossing it out there to get things rolling. If we end up doing it differently I don't really care, I'm not married to any solution but merely wish to solve a problem. If that ends up being solved differently, the outcome is the same to me.
-- Jens Axboe
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