Re: Can I see what device was used to mount btrfs?

[Andrei Borzenkov <arvidjaar@gmail.com>]

03.05.2017 14:26, Austin S. Hemmelgarn пишет:
> On 2017-05-02 15:50, Goffredo Baroncelli wrote:
>> On 2017-05-02 20:49, Adam Borowski wrote:
>>>> It could be some daemon that waits for btrfs to become complete.  Do we
>>>> have something?
>>> Such a daemon would also have to read the chunk tree.
>>
>> I don't think that a daemon is necessary. As proof of concept, in the
>> past I developed a mount helper [1] which handled the mount of a btrfs
>> filesystem:
>> this handler first checks if the filesystem is a multivolume devices,
>> if so it waits that all the devices are appeared. Finally mount the
>> filesystem.
>>
>>> It's not so simple -- such a btrfs device would have THREE states:
>>>
>>> 1. not mountable yet (multi-device with not enough disks present)
>>> 2. mountable ro / rw-degraded
>>> 3. healthy
>>
>> My mount.btrfs could be "programmed" to wait a timeout, then it mounts
>> the filesystem as degraded if not all devices are present. This is a
>> very simple strategy, but this could be expanded.
>>
>> I am inclined to think that the current approach doesn't fit well the
>> btrfs requirements.  The roles and responsibilities are spread to too
>> much layer (udev, systemd, mount)... I hoped that my helper could be
>> adopted in order to concentrate all the responsibility to only one
>> binary; this would reduce the interface number with the other
>> subsystem (eg systemd, udev).
> The primary problem is that systemd treats BTRFS like a block-layer
> instead of a filesystem (so it assumes all devices need to be present),
> and that it doesn't trust the kernel's mount function to work correctly.

My understanding is that before kernel mount can succeed for
multi-device btrfs, kernel must be made aware of devices that comprise
this filesystem. This is done by using (equivalent of) "btrfs device
scan" or "btrfs device ready". Am I wrong here?

>  As a result, it assumes that the mount operation will fail if it
> doesn't see all the devices instead of just trying it like it should.

So do you suggest that mount will succeed even if kernel is not made
aware of all devices? If not, could you elaborate how btrfs should be
mounted on boot - we must give mount command some device, right? How
should we chose this device?