Re: document ext3 requirements

[Pavel Machek <pavel@suse.cz>]

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
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