Re: Btrfs: wipe all the superblock [redhat bugzilla 889888]

[David Sterba <dsterba@suse.cz>]

On Sun, Jan 06, 2013 at 07:28:55PM +0100, Goffredo Baroncelli wrote:
> Currently wipefs doesn't clear all the superblock of btrfs. Only the first 
> one is cleared.
> 
> Btrfs has three superblocks. The first one is placed at 64KB, the second 
> one at 64MB, the third one at 256GB.

It can have as much as 4 superblock backup copies:

Superblock offset 0 is 65536 (0x10000, block=16/0x10)
Superblock offset 1 is 67108864 (0x4000000, block=16384/0x4000)
Superblock offset 2 is 274877906944 (0x4000000000, block=67108864/0x4000000)
Superblock offset 3 is 1125899906842624 (0x4000000000000, block=274877906944/0x4000000000)
Superblock offset 4 is 4611686018427387904 (0x4000000000000000, block=1125899906842624/0x4000000000000)

> If the first superblock is valid except that the "magic field" is zeroed,
> btrfs skips the check of the other superblocks.
> If the first superblock is fully invalid, btrfs checks for the other
> superblock.
> 
> So zeroing the first superblock "magic field" at the beginning seems
> that the filesystem is wiped. But when the first superblock is overwritten
> (e.g. by another filesystem), then the other two superblocks may be considered
> valid, and the filesystem may resurrect.

And for that purpose all the superblock copies should be taken into
account, regardless of the tricks that btrfs_mount applies.


david