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Date: Wed, 29 Nov 2023 21:54:02 +0100
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Reply-To: kreijack@inwind.it
Subject: Re: [PATCH RFC] btrfs: pick device with lowest devt for show_devname
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To: Anand Jain <anand.jain@oracle.com>, dsterba@suse.cz
Cc: linux-btrfs@vger.kernel.org
References: <85226cf68d7a72a034f0c0895b96b2557169755b.1698917826.git.anand.jain@oracle.com>
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From: Goffredo Baroncelli <kreijack@inwind.it>
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On 29/11/2023 00.28, Anand Jain wrote:
> 
> 
> On 28/11/2023 16:00, Goffredo Baroncelli wrote:
>> On 27/11/2023 12.48, Anand Jain wrote:
>>>
>>>
>>> On 11/25/23 09:09, Anand Jain wrote:
>> [...]
>>>> I am skeptical about whether we have a strong case to create a single
>>>> pseudo device per multi-device Btrfs filesystem, such as, for example
>>>> '/dev/btrfs/<fsid>-<random>/rootid=5' and which means pseudo device
>>>> will carry the btrfs-magic and the actual blk devices something else.
>>>>
>>>> OR for now, regarding the umount issue mentioned above, we just can
>>>> document it for the users to be aware of.
>>>>
>>>> Any feedback is greatly appreciated.
>>>>
>>>
>>> How about if we display the devices list in the options, so that
>>> user-land libs have something in the mount-table that tells all
>>> the devices part of the fsid?
>>>
>>> For example:
>>> $ cat /proc/self/mounts | grep btrfs
>>>
>>> /dev/sda1 /btrfs btrfs rw,relatime,space_cache=v2,subvolid=5,subvol=/,device=/dev/sda2,device=/dev/sdb3  0 0
>>>
>>
>> When I developed code to find a btrfs mount point from a disk, I had to
>> consider all the devices involved and check if one is in /proc/self/mounts.
>>
>> Putting the devices list as device=<xxx>,device=<yyy> doesn't change anything because
>> the code has to manage a btrfs filesistem as "special" in any case.
>> To get the map <btrfs-uuid> <-> <devices-list> I used libblkid.
[...]
> 
> Regarding libblkid for Btrfs device discovery, I m little confused what are you referring to, an example would be helpful.

I developed a little utility to build for each btrfs filesystem:
- all the devices involved
- all the mountpoint (if any) where the filesystem is mounted and the subvolume used as root.

It was nice because it got all these information only using:
- libblkid
- parsing /proc/self/mountinfo



> 
> Thanks.

-- 
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
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