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Subject: Re: WARNING: CPU: 3 PID: 439 at fs/btrfs/ctree.h:1559
 btrfs_update_device+0x1c5/0x1d0 [btrfs]
To: Rich Rauenzahn <rich@shroop.net>, Rich Rauenzahn <rrauenza@gmail.com>,
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From: Qu Wenruo <quwenruo.btrfs@gmx.com>
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Date: Thu, 21 Sep 2017 07:19:39 +0800
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On 2017年09月21日 02:10, Rich Rauenzahn wrote:
> 
> On 9/20/2017 9:58 AM, Rich Rauenzahn wrote:
>>
>> What's the most direct way to do that?  (Was about to risk breaking 
>> the mirror and repartitioning!  I'd rather not!)
>>
>> Hmm -- maybe this worked:
>>
>> $ sudo btrfs filesystem resize -1m /.MEDIA/
>> Resize '/.MEDIA/' of '-1m'
> No, doesn't seem to have worked.  I used btrfs fi show --raw /.MEDIA to 
> see the size in bytes, and then ...
> 
> sudo btrfs filesystem resize N:SIZE /.MEDIA/
> 

Yes, this is what I did to my fs.
While in fact you can do it better by using:

btrfs fi resize N:max <path>

Or

btrfs fi resize N:-1 <path>

The first one will resize the devid N to its max size.
The 2nd one will resize the devid N to current size - 1 (byte).

Both will go through shrink routine, which will do the round_down.

Thanks,
Qu

> Where N was the disk number in the show output, and SIZE was the number 
> of bytes to set it to that was a multiple of 4k.
> 
> Rich