Hi David,
On 01/07/2013 05:33 PM, David Sterba wrote:
> 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)
Are you sure ?
Regarding the btrfs-progs suite, I looked at the btrfs_read_dev_super():
[..]
for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
bytenr = btrfs_sb_offset(i);
ret = pread64(fd, &buf, sizeof(buf), bytenr);
Where BTRFS_SUPER_MIRROR_MAX is 3.
Regarding the kernel code, I looked at several function which call
btrfs_sb_offset(); everywhere there is an upper limit of the superblock
numbero which is BTRFS_SUPER_MIRROR_MAX, which is still 3.
Moreover I performed the following test:
$ ls -lh 7tb-filesystem.img
-rw-r--r-- 1 ghigo ghigo 7.1E Jan 7 18:49 7eb-filesystem.img
$ /sbin/mkfs.btrfs 7eb-filesystem.img
$ cat extract-sign.py
import os
BTRFS_SUPER_MIRROR_SHIFT = 12
BTRFS_SUPER_INFO_OFFSET = (64*1024)
def btrfs_sb_offset(mirror):
start = 16*1024
if(mirror):
return start << (BTRFS_SUPER_MIRROR_SHIFT * mirror)
return BTRFS_SUPER_INFO_OFFSET
f = open("7eb-filesystem.img","r")
for i in range(5):
pos = btrfs_sb_offset(i)+64
f.seek(pos)
sign = f.read(8)
print "Superblock #%d - %20d - '%s'"%(i,pos,sign)
$ python extract-sign.py
Superblock #0 - 65600 - '_BHRfS_M'
Superblock #1 - 67108928 - '_BHRfS_M'
Superblock #2 - 274877907008 - '_BHRfS_M'
Superblock #3 - 1125899906842688 - ''
Superblock #4 - 4611686018427387968 - ''
To me it seems that in a 7TB filesystem there is only 3 superblocks.
>
>> 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
>
--
gpg @keyserver.linux.it: Goffredo Baroncelli (kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5
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