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 -- To unsubscribe from this list: send the line "unsubscribe util-linux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
