At Thu, 19 Oct 2017 17:05:18 +0800,
Qu Wenruo wrote:
>
>
>
> On 2017年10月19日 16:34, Misono, Tomohiro wrote:
> > On 2017/10/19 16:45, Satoru Takeuchi wrote:
> >> Some tools can select which superblock these commands use by "-s
> >> <superblock>"
> >> option. Although this option says the valid values are 0-2, we can set 3
> >> if filesystem is very large.
> >>
> >
> > Hello,
> > Wiki says there are 4 superblocks. However in the implementation
> > BTRFS_SUPER_MIROR_MAX
> > is 3 and 0 indicates the block at 64K (disk-io.h of btrfs-progs), therefore
> > I think
> > there is no 4th superblock actually.
>
> Kernel implementation also shows that it will only update up to 3
> superblocks:
>
> ---
> if (max_mirrors == 0)
> max_mirrors = BTRFS_SUPER_MIRROR_MAX;
>
> for (i = 0; i < max_mirrors; i++) {
> bytenr = btrfs_sb_offset(i);
> if (bytenr + BTRFS_SUPER_INFO_SIZE >=
> device->commit_total_bytes)
> break;
> ---
>
> And BTRFS_SUPER_MIRROR_MAX is 3:
> ---
> #define BTRFS_SUPER_MIRROR_MAX 3
> ---
>
> So even you can set any value and btrfs_sb_offset() can calculate the
> super block offset, you will just read out some garbage.
My fault, sorry. I should read source more carefully. And thank you both
to let me know my mistake.
Thanks,
Satoru
>
> Thanks,
> Qu
> >
> > Regards,
> > Tomohiro
> >
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> >
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