Messages in this thread | | | Date | Wed, 9 Jul 2008 12:27:15 -0400 | From | Theodore Tso <> | Subject | Re: [RFC] ext3/jbd, kernel 2.6.13, make ext3 mountable as ext2 when journal is empty. |
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On Wed, Jul 09, 2008 at 05:24:25PM +0200, Jan Willem van den Brand wrote: > This patch makes ext3 mountable as ext2 even in case of a power > failure while mounted as ext3. We have tested it for kernel 2.6.13 but > it should be fairly easy to get it to work for other versions.
What's the rationale for doing this. Why is it *useful* to have the filesystem be mountable as ext2 in case of a power failure? The whole point of ext3 is to be able to keep the filesystem consistent in case of a power failure.
So the patch as given would never go into the kernel as-is, and in the general case it is totally counterproductive. Maybe it could go in as a optional behaviour enabled by a mount option, but that's assuming we can be convinced it's a good idea.
> In case of a power failure, the INCOMPAT flag is not reset. Systems > that suffer from frequent power failure (e.g. SD-cards that are > unsafely removed) will often not be mountable as ext2.
So what? Why can't you just run the journal and check the filesystem for consistency? If the user did a hot-eject while the SD-card was being written, even with your patch there is no guarantee that the card will even be readable. Some SD-cards go totally non-functional due to corruption at the flash translation layer when they are yanked in the middle of an update....
> Obviously, this solution will result in poor performance when many > small files are frequently closed after write but that is not the case > in our system (TomTom navigation device).
How often does your TomTom device need to update files? A better (userspace-only) solution might be keep the filesystem mounted read-only, and when you need to write to the filesystem, turn on the LED (which hopefully will be a hint to the users to keep their grubby little paws off the eject button), remount it read/write, do your file writes, then remount it read/only, and turn off the LED.
Regards,
- Ted
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