Re: [PATCH RFC] btrfs: Use backup superblocks if and only if the first superblock is valid but corrupted.

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 07/24/2014 05:28 PM, Chris Mason wrote:
> 
> 
> On 06/26/2014 11:53 PM, Qu Wenruo wrote:
>> Current btrfs will only use the first superblock, making the backup
>> superblocks only useful for 'btrfs rescue super' command.
>>
>> The old problem is that if we use backup superblocks when the first
>> superblock is not valid, we will be able to mount a none btrfs
>> filesystem, which used to contains btrfs but other fs is made on it.
>>
>> The old problem can be solved related easily by checking the first
>> superblock in a special way:
>> 1) If the magic number in the first superblock does not match:
>>    This filesystem is not btrfs anymore, just exit.
>>    If end-user consider it's really btrfs, then old 'btrfs rescue super'
>>    method is still available.
>>
>> 2) If the magic number in the first superblock matches but checksum does
>>    not match:
>>    This filesystem is btrfs but first superblock is corrupted, use
>>    backup roots. Just continue searching remaining superblocks.
> 
> I do agree that in these cases we can trust that the backup superblock
> comes from the same filesystem.
> 
> But, for right now I'd prefer the admin get involved in using the backup
> supers.  I think silently using the backups is going to lead to surprises.
Maybe there could be a mount non-default mount-option to use backup
superblocks iff the first one is corrupted, and then log a warning
whenever this actually happens?  Not handling stuff like this
automatically really hurts HA use cases.



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