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    SubjectRe: raid is dangerous but that's secret (was Re: [patch] ext2/3:
    >> An embedded checksum, no matter how good, can't tell you if
    >> the data is stale; you need a way to distinguish versions in the pointer.

    > I would disagree with that.
    > If the embedded checksum is a function of both the data and the address
    > of the data (in whatever address space seems most appropriate) then it can
    > still verify that the data found with the checksum is the data that was
    > expected.
    > And storing the checksum with the data (where it is practical) means
    > index blocks can be more dense so on average fewer accesses to storage
    > are needed.

    I must not have been clear. Originally, block 100 has contents version 1.
    This includes a correctly computed checksum.

    Then you write version 2 of the data there. But there's a bit error in
    the address and the write goes to block 256+100 = 356. So block
    100 still has the version 1 contents, complete with valid checksum.
    (Yes, block 356 is now corrupted, but perhaps it's not even allocated.)

    Then we go to read block 100, find a valid checksum, and return incorrect
    data. Namely, version 1 data, when we expact and want version 2.

    Basically, the pointer has to say which *version* of the data it points to,
    not just the block address. Otherwise, it can't detect a missing write.

    If density is a big issue, then including a small version field is a

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      Last update: 2009-09-01 13:21    [W:0.019 / U:110.704 seconds]
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