Messages in this thread | | | From | John Bradford <> | Subject | Re: Are linux-fs's drive-fault-tolerant by concept? | Date | Mon, 21 Apr 2003 10:55:12 +0100 (BST) |
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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.
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