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

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Jan 07, 2013 at 07:20:16PM +0100, Goffredo Baroncelli wrote:
> 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'

64 KiB

> Superblock #1 -             67108928 - '_BHRfS_M'

256 MiB

> Superblock #2 -         274877907008 - '_BHRfS_M'

1 TiB

> Superblock #3 -     1125899906842688 - ''

4 PiB

> Superblock #4 -  4611686018427387968 - ''

16 EiB


> To me it seems that in a 7TB filesystem there is only 3 superblocks.

   That would be as expected. How many on a 5 PiB filesystem, though?
Or a 20 EiB one?

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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  --- "He's a nutcase, you know. There's no getting away from it -- ---  
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