On 05/19/2014 02:54 AM, Chris Murphy wrote:
> Summary:
>
> It's insufficient to pass rootflags=degraded to get the system root
> to mount when a device is missing. It looks like when a device is
> missing, udev doesn't create the dev-disk-by-uuid linkage that then
> causes systemd to change the device state from dead to plugged. Only
> once plugged, will systemd attempt to mount the volume. This issue
> was brought up on systemd-devel under the subject "timed out waiting
> for device dev-disk-by\x2duuid" for those who want details.
>
[...]
>
> I think the key problem is either a limitation of udev, or a problem
> with the existing udev rule, that prevents the link creation for any
> remaining btrfs device. Or maybe it's intentional. But I'm not a udev
> expert. This is the current udev rule:
>
> # cat /usr/lib/udev/rules.d/64-btrfs.rules
> # do not edit this file, it will be overwritten on update
>
> SUBSYSTEM!="block", GOTO="btrfs_end" ACTION=="remove",
> GOTO="btrfs_end" ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
>
> # let the kernel know about this btrfs filesystem, and check if it is complete
> IMPORT{builtin}="btrfs ready $devnode"
>
> # mark the device as not ready to be used by the system
> ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"
>
> LABEL="btrfs_end"
The key is the line
IMPORT{builtin}="btrfs ready $devnode"
This line sets ID_BTRFS_READY=0 if a filesystem is not ready; otherwise
set ID_BTRFS_READY=1 [1].
The next line
ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"
sets SYSTEMD_READY=0 if the filesystem is not ready so the "plug" event
is not raised to systemd.
This is my understanding.
> How this works with raid:
>
> RAID assembly is separate from filesystem mount. The volume UUID
> isn't available until the RAID is successfully assembled.
>
> On at least Fedora (dracut) systems with the system root on an md
> device, the initramfs contains 30-parse-md.sh which includes a loop
> to check for the volume UUID. If it's not found, the script sleeps
> for 0.5 seconds, and then looks for it again, up to 240 times. If
> it's still not found at attempt 240, then the script executes mdadm
> -R to forcibly run the array with fewer than all devices present
> (degraded assembly). Now the volume UUID exists, udevd creates the
> linkage, systemd picks this up and changes device state from dead to
> plugged, and then executes a normal mount command.
> The approximate Btrfs equivalent down the road would be a similar
> initrd script, or maybe a user space daemon, that causes btrfs device
> ready to confirm/deny all devices are present. And after x number of
> failures, then it's issue an equivalent to mdadm -R which right now
> we don't seem to have.
I suggest to implement a mount.btrfs command, which waits all the
needed disks until a timeout expires. After this timeout it could try
a "degraded" mount until a second timeout. Only then it fails.
Each time a device appear, the system may start mount.btrfs. Each
invocation has to test if there is another instance of mount.btrfs related
to the same filesystem; if so it ends, otherwise it follows the above
behavior.
>
> That equivalent might be a decoupling of degraded as a mount option,
> such that the user space tool deals with degradedness. And the mount
>[...]
>
> Chris Murphy
G.Baroncelli
[1] http://lists.freedesktop.org/archives/systemd-commits/2012-September/002503.html
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