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    SubjectRe: [patch] ext2/3: document conditions when reliable operation is possible
    On 08/27/2009 04:51 PM, Rob Landley wrote:
    > On Thursday 27 August 2009 06:43:49 Ric Wheeler wrote:
    >
    >> On 08/26/2009 11:53 PM, Rob Landley wrote:
    >>
    >>> On Tuesday 25 August 2009 18:40:50 Ric Wheeler wrote:
    >>>
    >>>> Repeat experiment until you get up to something like google scale or the
    >>>> other papers on failures in national labs in the US and then we can have
    >>>> an informed discussion.
    >>>>
    >>> On google scale anvil lightning can fry your machine out of a clear sky.
    >>>
    >>> However, there are still a few non-enterprise users out there, and
    >>> knowing that specific usage patterns don't behave like they expect might
    >>> be useful to them.
    >>>
    >> You are missing the broader point of both papers.
    >>
    > No, I'm dismissing the papers (some of which I read when they first came out
    > and got slashdotted) as irrelevant to the topic at hand.
    >

    I guess I have to dismiss your dismissing then.
    > Pavel has two failure modes which he can trivially reproduce. The USB stick
    > one is reproducible on a laptop by jostling said stick. I myself used to have
    > a literal USB keychain, and the weight of keys dangling from it pulled it out
    > of the USB socket fairly easily if I wasn't careful. At the time nobody had
    > told me a journaling filesystem was not a reasonable safeguard here.
    >
    > Presumably the degraded raid one can be reproduced under an emulator, with no
    > hardware directly involved at all, so talking about hardware failure rates
    > ignores the fact that he's actually discussing a _software_ problem. It may
    > happen in _response_ to hardware failures, but the damage he's attempting to
    > document happens entirely in software.
    >
    > These failure modes can cause data loss which journaling can't help, but which
    > journaling might (or might not) conceivably hide so you don't immediately
    > notice it. They share a common underlying assumption that the storage
    > device's update granularity is less than or equal to the filesystem's block
    > size, which is not actually true of all modern storage devices. The fact he's
    > only _found_ two instances where this assumption bites doesn't mean there
    > aren't more waiting to be found, especially as more new storage media types
    > get introduced.
    >
    > Pavel's response was to attempt to document this. Not that journaling is
    > _bad_, but that it doesn't protect against this class of problem.
    >
    > Your response is to talk about google clusters, cloud storage, and cite
    > academic papers of statistical hardware failure rates. As I understand the
    > discussion, that's not actually the issue Pavel's talking about, merely one
    > potential trigger for it.
    >
    > Rob
    >




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