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    SubjectRe: writing file to disk: not as easy as it looks

    On Wed, 3 Dec 2008, Theodore Tso wrote:

    > > Ok, "memory failed before disk" is ... bad hardware.
    > It's PC class hardware. Live with it. Back when SGI made their own
    > hardware, they noticed this problem, and so they wired up their SGI
    > machines with powerfail interrupts, and extra big capacitors in their
    > power supplies, and when Irix got a powerfail interrupt, it would
    > frantically run around aborting DMA transfers to avoid this particular
    > problem. At least, that's what an old-timer SGI engineer (who is
    > unfortunately no longer at SGI) told me.

    I heard this too --- I just don't understand why did they route it to an
    interrupt and undertook the complicated sequence of aborting the commands
    by the kernel --- instead of simply routing it to PCI reset line --- that
    would reset the controller and stop it from feeding data to disks.

    Also, if they had ECC memory, the chipset should detect unrecoverable
    garbage and respond with target-abort or full system reset and not feed
    bad data to the controller.

    > PC class hardware don't have power fail interrupts. Hence, my advice
    > to you is that if you use a filesystem that does logical journalling
    > --- better have a UPS.

    ATX has PWR_OK pin that should be deasserted on power failure before the
    voltage drops.

    I don't know if motherboards use it --- but there should be no problem
    routing the pin to the chipset reset and stop it before power goes low.

    > > ...but... you seem to be saying that modern filesystems can damage
    > > data even on "sane" hardware.
    > The example I gave was one where a disk failure could cause a file
    > that had previously been sucessfully written to disk and fsync()'ed to
    > be damaged by another filesystem operation ***in the face of hard
    > drive failure***. Surely that is obvious. The most obvious case of
    > that might be if the disk controller gets confused and slams a data
    > block into the wrong location on disk (there's a reason why DIF
    > includes the sector number in its checksum and why some enterprise
    > databases do the same thing in their tablespace blocks --- it happens
    > often enough that paranoid data integrity engineers worry about it).

    You can read the block number back from ATA disk after you write it and
    before you submit the command.


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