Re: [patch] ext2/3: document conditions when reliable operation is possible

[Ric Wheeler <rwheeler@redhat.com>]

On 08/25/2009 06:58 PM, Neil Brown wrote:
> On Monday August 24, tytso@mit.edu wrote:
>> On Mon, Aug 24, 2009 at 11:25:19PM +0200, Pavel Machek wrote:
>>>> I have to admit that I have not paid enough attention to this specifics
>>>> of your ext3 + flash card issue - is it the ftl stuff doing out of order
>>>> IO's?
>>>
>>> The problem is that flash cards destroy whole erase block on unplug,
>>> and ext3 can't cope with that.
>>
>> Sure --- but name **any** filesystem that can deal with the fact that
>> 128k or 256k worth of data might disappear when you pull out the flash
>> card while it is writing a single sector?
>
> A Log structured filesystem could certainly be written to deal with
> such a situation, providing by 'deal with' you mean 'only loses data
> that has not yet been acknowledged to the application'.  Of course the
> filesystem would need clear visibility into exactly how these blocks
> are positioned.
>
> I've been playing with just such a filesystem for some time (never
> really finding enough time) with the goal of making it work over RAID5
> with no data risk due to power loss.  One day it will be functional
> enough for others to try....
>
> It is entirely possible that NILFS could be made to meet that
> requirement, but I haven't made time to explore NILFS so I cannot be
> sure.
>
> NeilBrown
>

I am not sure that log structure will protect you from this scenario since once 
you clean the log, the non-logged data is assumed to be correct.

If your cheap flash storage device can nuke random regions of that clean 
storage, you will lose data....

ric