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

    >>> Sure --- but name **any** filesystem that can deal with the fact that
    >>> 128k or 256k worth of data might disappear when you pull out the flash
    >>> card while it is writing a single sector?
    >> First... I consider myself quite competent in the os level, yet I did
    >> not realize what flash does and what that means for data
    >> integrity. That means we need some documentation, or maybe we should
    >> refuse to mount those devices r/w or something.
    >> Then to answer your question... ext2. You expect to run fsck after
    >> unclean shutdown, and you expect to have to solve some problems with
    >> it. So the way ext2 deals with the flash media actually matches what
    >> the user expects. (*)
    > you loose data in ext2


    >> OTOH in ext3 case you expect consistent filesystem after unplug; and
    >> you don't get that.
    > the problem is that people have been preaching that journaling
    > filesystems eliminate all data loss for no cost (or at worst for minimal
    > cost).
    > they don't, they never did.
    > they address one specific problem (metadata inconsistancy), but they do
    > not address data loss, and never did (and for the most part the
    > filesystem developers never claimed to)

    Well, in case of flashcard and degraded MD Raid5, ext3 does _not_
    address metadata inconsistency problem. And that's why I'm trying to
    fix the documentation. Current ext3 documentation says:

    #Journaling Block Device layer
    #The Journaling Block Device layer (JBD) isn't ext3 specific. It was
    #to add journaling capabilities to a block device. The ext3 filesystem
    #will inform the JBD of modifications it is performing (called a
    #The journal supports the transactions start and stop, and in case of a
    #the journal can replay the transactions to quickly put the partition
    #back into
    #a consistent state.

    There's no mention that this does not work on flash cards and degraded
    MD Raid5 arrays.

    > people somehow have the expectation that ext3 does the data equivalent of
    > solving world hunger, it doesn't, it never did, and it never claimed
    > to.

    It claims so, above.

    > personally I don't consider the two filesystems to be significantly
    > different in terms of the data loss potential. I think people are more
    > aware of the potentials with XFS than with ext3, but I believe that the
    > risk of loss is really about the same (and pretty much for the same
    > reasons)

    Ack here.

    >> Again, ext2 handles that in a way user expects it.
    >> At least I was teached "ext2 needs fsck after powerfail; ext3 can
    >> handle powerfails just ok".
    > you were teached wrong. the people making these claims for ext3 didn't
    > understand what ext3 does and doesn't do.

    Cool. So... can we fix the documentation?
    (cesky, pictures)

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      Last update: 2009-08-25 11:35    [W:0.025 / U:5.868 seconds]
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