Re: [PATCH 2/2] btrfs: sysfs add devinfo/fsid to retrieve fsid from the device

[Anand Jain <anand.jain@oracle.com>]

On 21/10/2021 12:03, Anand Jain wrote:
> On 21/10/2021 02:59, David Sterba wrote:
>> On Tue, Oct 19, 2021 at 08:22:10AM +0800, Anand Jain wrote:
>>> In the case of the seed device, the fsid can be different from the 
>>> mounted
>>> sprout fsid.  The userland has to read the device superblock to know the
>>> fsid but, that idea fails if the device is missing. So add a sysfs
>>> interface devinfo/<devid>/fsid to show the fsid of the device.
>>>
>>> For example:
>>>   $ cd /sys/fs/btrfs/b10b02a5-f9de-4276-b9e8-2bfd09a578a8
>>>
>>>   $ cat devinfo/1/fsid
>>>   c44d771f-639d-4df3-99ec-5bc7ad2af93b
>>>   $ cat  devinfo/3/fsid
>>>   b10b02a5-f9de-4276-b9e8-2bfd09a578a8
>>
>> How do you create such setup? I can't reproduce it.
>>
>> The simplest seeding:
>>
>>    mkfs.btrfs /dev/sdx
>>    btrfstune -S 1 /dev/sdx
>>    mount /dev/sdx mnt
>>    ... the device has the same FSID as is the sysfs directory name
>>
>> With a new device and removed the seeding one:
>>
>>    btrfs device add /dev/sdy mnt
> 
> At this step, we generate a new fsid for the writeable FS. Let's call
> it sprout-fsid. (If you check the sys-fs fsid, you will have two fsid
> here).
> 
> Also, at this step,
> the
>      fs_info->fs_devices->fsid
> changes from seed-fsid to the sprout-fsid.
> So, we should make the <mnt> also a writeable without the need to call
> 'remount,rw' explicitly IMO. What do you think?
> 

More importantly, the fs_info->super_copy is of sprout device.
So there isn't any reason that we should maintain the new
sprout fs as readonly after the device add.


>>    mount -o remount,rw mnt
>>    btrfs device delete /dev/sdx mnt
>>    ... both devices have the same fsid as well 
> 
> 
> Thanks, Anand
> 
> 
> 
>