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

[Goffredo Baroncelli <kreijack@inwind.it>]

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).

For example, it would be possible to implement a sane check that prevent to mount a btrfs filesystem if two devices exposes the same UUID... 


BR
G.Baroncelli

[1] See "[RFC][PATCH v2] mount.btrfs helper" thread ( https://marc.info/?l=linux-btrfs&m=141736989508243&w=2 )



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