Re: [PATCH v2] Btrfs: remove superblock writing after fatal error

[Liu Bo <liub.liubo@gmail.com>]

On 08/02/2012 10:28 PM, Jan Schmidt wrote:
> On Thu, August 02, 2012 at 15:46 (+0200), Arne Jansen wrote:
>> On 02.08.2012 13:57, Liu Bo wrote:
>>> Anyway, for now, our error flag has only been stored in memory, so what
>>> about just keep it until we find a graceful way?
>>
>> Yeah, we need this patch to restore consistency. We can define a fixed
>> area on disk (e.g. behind the superblock) where we can write the flag
>> to without risking the superblock.
> 
> At least we all agree that we need this patch, fine.
> 
> We don't yet agree that we need a place to store a "please consider fsck" flag.
> Can I please get one concrete example in which situation we
> 
> a) do detect the user should really do a file system check AND
> b) do not abort the transaction to clean the mess up?
> 
> (An example on how we could fail transaction cleanup is also accepted).
> 

Unfortunately I don't have such an example either.

Since we always get COW on metadata, I believe that it's ok to just roll
back on failure.

> If such a situation doesn't exist, there's no need for this flag. The fact that
> ext has such a flag doesn't convince me, probably because I know nothing about
> ext. I can imagine that they can detect file system errors without the ability
> to return to a potentially older consistent state.
> 

This error flag is also used to indicate filesystem's error state for
transaction cleanup, so keeping it in memory is reasonable.

thanks,
liubo

> Thanks,
> -Jan
>