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    SubjectRe: 2.6.36 io bring the system to its knees
    On Wed, Nov 10, 2010 at 09:33:29AM -0500, Theodore Tso wrote:
    > On Nov 9, 2010, at 8:32 PM, Dave Chinner wrote:
    > > Don't forget to mention data=writeback is not the default because if
    > > your system crashes or you lose power running in this mode it will
    > > *CORRUPT YOUR FILESYSTEM* and you *WILL LOSE DATA*. Not to mention
    > > the significant security issues (e.g stale data exposure) that also
    > > occur even if the filesystem is not corrupted by the crash. IOWs,
    > > data=writeback is the "fast but I'll eat your data" option for ext3.
    > This is strictly speaking not true. Using data=writeback will not
    > cause you to lose any data --- at least, not any more than you
    > would without the feature. If you have applications that write
    > files in an unsafe way, that data is going to be lost, one way or
    > another. (i.e., with XFS in a similar situation you'll get a
    > zero-length file) The difference is that in the case of a system
    > crash, there may be unwritten data revealed if you use
    > data=writeback. This could be a security exposure, especially if
    > you are using your system in as time-sharing system, and where you
    > see the contents of deleted files belonging to another user.

    In theory, that's all that is _supposed_ to happen. However, my
    recent experience is that massive ext3 filesystem corruption occurs
    in data=writeback mode when the system crashes and that does not
    happen in ordered mode.

    Why do you think i posted the patches to change the default back to
    ordered mode a few months back? I basically trashed the root ext3
    partitions on three test machines (to the point where >5000 files
    across /sbin, /bin, /lib and /usr were corrupted or missing and I
    had to reinstall from scratch) when I'd forgotten to set the
    ordered-is-defult config option in the kernel i was testing. And
    that is when the only thing being written to the root filesystems
    was log files...

    The worst part about this was that I also had ext3 filesystems
    corrupted by crashes in such a way that e2fsck didn't detect it but
    they would repeatedly trigger kernel crashes at runtime....

    > So it is not an "eat your data" situation,

    My experience says otherwise....


    Dave Chinner

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