Re: BTRFS_IOC_DEVICES_READY and removed devices

[Josef Bacik <jbacik@fusionio.com>]

On Mon, Sep 17, 2012 at 04:46:08PM -0600, Kay Sievers wrote:
> We are currently playing around with native btrfs multi-device support
> in systemd. We already committed the needed pieces to systemd git, to
> register all detected btrfs filesystems with the kernel.
> 
> For volumes which are listed in fstab for mounting, we delay the
> actual mount-attempt of a multi-device volume until we see READY
> returned from BTRFS_IOC_DEVICES_READY. A line with UUID= in /etc/fstab
> with "nofail" in the options field, and we can boot up without any
> device plugged in. Now plugging in devices one-after-the-other until
> the volume has a full tree of devices; with the last device there,
> systemd just mounts the volume as expected.
> 
> This seems to work very well so far, unless a device which is already
> registered disappears, which is a kind of valid hotplug scenario we
> should handle better:
> 
> If one device of a 2-device volume is registered with the in-kernel
> cache, and then the device is unplugged from the system, the cache
> state does not get updated. If then the other device of the 2-device
> volume is registered, BTRFS_IOC_DEVICES_READY indicates ready; but in
> fact only one of two needed devices are available at that time, and
> mounting fails.
> 
> Can we somehow subscribe to device media-changes/removal to prevent
> the stale device state in the in-kernel cache?
> 
> Or alternatively make BTRFS_IOC_DEVICES_READY re-validate all involved
> block devices before it returns READY?

Hrm excellent question, I'll look at what options we have in kernel to check and
see if a device is still present and figure something out.  Thanks,

Josef