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    SubjectRe: Are linux-fs's drive-fault-tolerant by concept?
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    > > > > > > Fault tolerance in a filesystem layer means in practical terms
    > > > > > > that you are guessing what a filesystem should look like, for the
    > > > > > > disk doesn't answer that question anymore. IMHO you don't want
    > > > > > > that to be done automagically, for it might go right sometimes,
    > > > > > > but also might trash everything on RW filesystems.
    > > > > >
    > > > > > Let me clarify again: I don't want fancy stuff inside the filesystem
    > > > > > that magically knows something about right-or-wrong. The only _very
    > > > > > small_ enhancement I would like to see is: driver tells fs there is an
    > > > > > error while writing a certain block => fs tries writing the same
    > > > > > data onto another block. That's it, no magic, no RAID
    > > > > > stuff. Very simple.
    > > > >
    > > > > That doesn't belong in the filesystem.
    > > > >
    > > > > Imagine you have ten blocks free, and you allocate data to all of them
    > > > > in the filesystem. The write goes to cache, and succeeds.
    > > > >
    > > > > 30 seconds later, the write cache is flushed, and an error is reported
    > > > > back from the device.
    > > >
    > > > And where's the problem?
    > > > Your case:
    > > > Immediate failure. Disk error.
    > > >
    > > > My case:
    > > > Immediate failure. Disk error (no space left for replacement)
    > > >
    > > > There's no difference.
    > >
    > > In my case, the machine can continue as normal. The filesystem is
    > > intact, (with no blocks free). The block device driver has to cope
    > > with the error, which could be as simple as holding the data in RAM
    > > until an operator has been paged to replace the disk.
    >
    > Forgive my ignorance, but I have not seen a case up to today where
    > ide, aicX or 3ware has called me up for a replacement unit, written
    > to it and been ok afterwards. What the heck are you talking of?

    Modern disks error correct on write. The user doesn't even know that
    they are doing it. If a disk actually reports back a write error, it
    is usually very broken.

    Incidentally, I don't see how your idea would even be implementable
    without disabling write caching.

    > I am not really interested in what a low-level driver could do
    > unless there is none that does it...

    I assume you mean 'one that does it'.

    If nobody was interested in what a low-level driver could do unless
    there was one that does it already, we wouldn't be innovating anything
    new.

    > And again, how do you think this should work out on your _root_
    > partition? (see below)

    1. Hot plug a new disk
    2. Duplicate the read-only root file system on to it
    3. Pivot root

    > > In your case, the filesystem is no longer in a usable state.
    >
    > I have yet to see an fs thats in a writeable state after the medium
    > is full ...

    It is perfectly writable, for example, for a delete operation.

    > > If that
    > > was the root filesystem, the machine will, at best, probably go in to
    > > single user mode, with a read-only root filesystem.
    >
    > How come?

    In my opinion, that would be the best course of action, if the device
    holding the root filesystem is faulty.

    > > > Thing is: If there are 11 blocks free and not ten, then you fail
    > >
    > > Wrong. See above.
    >
    > Please tell me when you were last "paged to replace the disk"? If you can't
    > tell me, then you know I am right by now.

    I have never been paged to replace a disk by a Linux system.

    That is why I would like to see this functionality added to Linux.

    John.
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