Re: document ext3 requirements

[Rob Landley <rob@landley.net>]

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.

Rob