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? Thanks, Kay -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
