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    SubjectRe: document ext3 requirements
    On Sun 2009-01-04 13:49:49, Rob Landley wrote:
    > On Saturday 03 January 2009 06:38:15 Pavel Machek wrote:
    > > +Ext3 expects disk/storage subsystem to behave sanely. On sanely
    > > +behaving disk subsystem, data that have been successfully synced will
    > > +stay on the disk. Sane means:
    > > +
    > > +* writes to media never fail. Even if disk returns error condition during
    > > + write, ext3 can't handle that correctly, because success on fsync was
    > > already + returned when data hit the journal.
    > > +
    > > + (Fortunately writes failing are very uncommon on disks, as they
    > > + have spare sectors they use when write fails.)
    > > +
    > > +* either whole sector is correctly written or nothing is written during
    > > + powerfail.
    > > +
    > > + (Unfortuantely, none of the cheap USB/SD flash cards I seen do behave
    > > + like this, and are unsuitable for ext3.
    >
    > Want to document the granularity issues with flash, while you're at it?
    >
    > An inherent problem with using flash as a normal block device is that the
    > flash erase size is bigger than most filesystem sector sizes. So when you
    > request a write, it may erase and rewrite the next 64k, 128k, or even a couple
    > megabytes on the really _big_ ones.
    >
    > If you lose power in the middle of that, ext3 won't notice that data in the
    > "sectors" _after_ the one your were trying to write to got trashed.
    >
    > The flash filesystems take this into account as part of their wear levelling
    > stuff (they normally copy the entire chunk into a new chunk, leaving the old
    > one in place until it's no longer needed), but they need to query the device
    > to get the erase granularity in order to do that, which is why they don't work
    > on non-flash block devices.

    Is there linux filesystem that can handle that? I know jffs2, but
    that's unsuitable for stuff like USB thumb drives, right?

    Does this sound like a fair summary?

    Sector writes are atomic (ATOMIC-SECTORS)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Either whole sector is correctly written or nothing is written during
    powerfail.

    Unfortuantely, none of the cheap USB/SD flash cards I seen do
    behave like this, and are unsuitable for all linux filesystems
    I know.

    An inherent problem with using flash as a normal block
    device is that the flash erase size is bigger than
    most filesystem sector sizes. So when you request a
    write, it may erase and rewrite the next 64k, 128k, or
    even a couple megabytes on the really _big_ ones.

    If you lose power in the middle of that, filesystem
    won't notice that data in the "sectors" _after_ the
    one your were trying to write to got trashed.

    Pavel
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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