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    SubjectRe: [patch] ext2/3: document conditions when reliable operation is possible
    Hi Theodore,

    thanks for the insightful writing.

    On 08/24/2009 04:01 PM, Theodore Tso wrote:

    ...snip ...

    > It's for this reason that I've never been completely sure how useful
    > Pavel's proposed treatise about file systems expectations really are
    > --- because all storage subsystems *usually* provide these guarantees,
    > but it is the very rare storage system that *always* provides these
    > guarantees.

    There is a thing called eMMC (embedded MMC) in the embedded world. You
    may consider it as a non-removable MMC. This thing is a block device from
    the Linux POW, and you may mount ext3 on top of it. And people do this.

    The device seems to have a decent FTL, and does not look bad.

    However, there are subtle things which mortals never think about. In
    case of eMMC - power cuts may make some sectors unreadable - eMMC returns
    ECC errors on reads. Namely, the sectors which were being written at
    the very moment when the power cut happened may become unreadable.
    And this makes ext3 refuse mounting the file-system, this makes
    chkfs.ext3 refuse the file-system. Although this should be fixable in
    SW, but we did not find time to do this so far.

    Anyway, my point is that documenting subtle things like this is a very
    good thing to do, just because nowadays we are trying to use existing
    software with flash-based storage devices, which may violate these
    subtle assumptions, or introduce other ones.

    Probably, Pavel did too good job in generalizing things, and it could be
    better to make a doc about HDD vs SSD or HDD vs Flash-based-storage.
    Not sure. But the idea to document subtle FS assumption is good, IMO.

    Best Regards,
    Artem Bityutskiy (Артём Битюцкий)
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