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    SubjectRe: ext2/3: document conditions when reliable operation is possible
    On Mon 2009-03-16 15:45:36, Greg Freemyer wrote:
    > On Thu, Mar 12, 2009 at 5:21 AM, Pavel Machek <pavel@ucw.cz> wrote:
    > <snip>
    > > +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" _around_ the
    > > +               one your were trying to write to got trashed.
    >
    > I had *assumed* that SSDs worked like:
    >
    > 1) write request comes in
    > 2) new unused erase block area marked to hold the new data
    > 3) updated data written to the previously unused erase block
    > 4) mapping updated to replace the old erase block with the new one
    >
    > If it were done that way, a failure in the middle would just leave the
    > SSD with the old data in it.

    The really expensive ones (Intel SSD) apparently work like that, but I
    never seen one of those. USB sticks and SD cards I tried behave like I
    described above.
    Pavel
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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