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Date: Thu, 19 Oct 2017 18:38:44 +0900
Message-ID: <87a80npjdn.wl-satoru.takeuchi@gmail.com>
From: Satoru Takeuchi <satoru.takeuchi@gmail.com>
To: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: "Misono, Tomohiro" <misono.tomohiro@jp.fujitsu.com>,
        Satoru Takeuchi <satoru.takeuchi@gmail.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs-progs: change mans to describe the third copy of superblock
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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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