allow mounting w/crc-checking disabled? (was Re: filesystem dead, xfs_repair won't help)

[L A Walsh <xfs@tlinx.org>]

Avi Kivity wrote:
> Today my kernel complained that in memory metadata is corrupt and
> asked that I run xfs_repair.  But xfs_repair doesn't like the
> superblock and isn't able to find a secondary superblock.
>   
Why doesn't xfs have an option to mount with metadata checksumming
disabled so people can recover their data?

Seems like it should be easy to provide, no?

Or rather, if a disk is created with the crc option, is it possible
to later switch it off or mount it without with checking disabled?

Yes, I know the mantra is that they should have had backups, but
in practice it's seems not the case in a majority of uses outside
of enterprise usage.  It sure seems that disabling a particular file
or directory (if necessary) affected by a bad-crc, would be
preferable to losing the whole disk.  That said, how many crc
errors would be likely to make things unreadable or inaccessible?
Given that the default before crc-checking was that the disks
were still usable (often with no error being flagged or noticed),
I'd suspect that the crc-checking is causing many errors to be
be flagged that before wouldn't have even been noticed. 

Overall I'm wondering if the crc option won't cause more disk-losses
than would occur without the option.  Or, in other words, it seems
that since crc-checking seems to cause the disk to be lost, turning
on crc checking is almost guaranteed to cause a higher incidence of
data loss if it can't be disable.