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    SubjectRe: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
    On Wed, Jan 16, 2008 at 09:02:50PM -0500, Daniel Phillips wrote:
    >
    > Have you observed that in the wild? A former engineer of a disk drive
    > company suggests to me that the capacitors on the board provide enough
    > power to complete the last sector, even to park the head.
    >

    The problem isn't with the disk drive; it's from the DRAM, which tend
    to be much more voltage sensitive than the hard drives --- so it's
    quite likely that you could end up DMA'ing garbage from the memory.
    In fact the fact that the disk drives lasts longer due to capacitors
    on the board, rotational inertia of the platters, etc., is part of the
    problem.

    It was observed in the wild by SGI, many years ago on their hardware.
    They later added extra capacitors on the motherboard and a powerfail
    interrupt which caused the Irix to run around frantically shutting
    down DMA's for a controlled shutdown. Of course, PC-class hardware
    has none of this. My source for this was Jim Mostek, one of the
    original Linux XFS porters. He had given me source code to a test
    program that would show this; basically zeroed out a region of disk,
    then started writing series of patterns on that part of the, and you
    you kicked out the power cord, and then see if there was any garbage
    on the disk. If you saw something that wasn't one of the patterns
    being written to the disk, then you knew you had a problem. I can't
    find the program any more, but it wouldn't be hard to write.

    I do know that I have seen reports from many ext2 users in the field
    that could only be explained by the hard drive scribbling garbage onto
    the inode table. Ext3 solves this problem because of its physical
    block journaling.

    - Ted


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