Re: umount and findmnt commands not working with btrfs labels ...

[George Mitchell <george@chinilu.com>]

So what I think I hear you saying on this issue is that though I can 
easily turn a label into a mount point with my little grep trick, umount 
is unable to do that so that in my scripts I will always have to use 
this kluge for things to work correctly?  And that of course the same 
would hold true with findmnt?

On 05/09/2013 02:42 AM, Karel Zak wrote:
> On Tue, May 07, 2013 at 07:48:40AM -0700, George Mitchell wrote:
>> problem.  Is there a different debug statement I can use with findmnt?  What
>   LIBMOUNT_DEBUG=  for libmount
>   LIBBLKID_DEBUG=  for libblkid
>
>   We usually use /dev/disk/by-label and by-uuid/ to convert LABELs and
>   UUIDs to device names. These symlinks are maintained by udevd and
>   created according to result from
>
>     blkid -o udev -p <device>
>
>   the duplicate LABELs are ignored, there is only one LABEL of course.
>
>> fix or whether I will need to redo this with each reboot?  Also you did your
>> testing with a simple one partition btrfs volume.  My system involves up to
>   I had two independent partitions, each initialized by mkfs.btrfs.
>
>> five partitions in a single RAID 1 volume.  My boot volume is spread over
>> two partitions.  I am wondering if that could have something to do with it?
>> In any case, here is what I come up with using the same debug prefix with
>> findmnt.
>   Ah, do you mean btrfs raid? For example:
>
>     mkfs.btrfs --data raid1 --label FOO /dev/sdd1 /dev/sdd2
>
>   then the LABEL and UUID is really duplicate.
>
>
>   Anyway, the right way how to umount any filesystem is to specify the
>   filesystem by mountpoint, for example
>
>     umount /mnt
>
>   this is the way how Linux umount(2) syscall works, because everything
>   else is unreliable. Don't forget that you can mount the same filesystem
>   on more places and sometimes filesystem != device (e.g. btrfs), etc.
>
>   The umount-by-device is marked in the umount(8) man page as obsolete.
>   It seems I have to add a note about RAIDs too.
>
>> Using findmnt on MAGEIA3BTR-BOOT produces the following.  It appears to find
>> the mount point and then print the result:
>>
>> [root@localhost ghmitch]# LIBMOUNT_DEBUG=0xffff findmnt
>> LABEL=MAGEIA3BTR-BOOT
> [...]
>> However, when I do the same operation with MAGEIA3BTR, it DOES seem to find
>> the mount point (4814: libmount:    CACHE: [0x861c8a0]: add entry [ 1]
>> (tag): /dev/sdd1: LABEL).  The problem is, that is NOT the mount point.  The
>> mount point is /dev/sde1.
>   It seems that we need a note about duplicate LABELs to the man page ;-)
>
>   Note, for Linux RAID (mdadm(8)) we're able to detect that the
>   filesystem is within a raid member device (e.g. /dev/sda<N>) and the
>   device is reported as RAID member, the filesystem is ignored. The
>   filesystem is visible only on the final raid device (e.g /dev/md0).
>
>   It means that the duplicate filesystems (RAID1 members) are invisible.
>
>
>   I guess something like this is unnecessary for btrfs, because you can
>   mount arbitrary btrfs raid member (device) as btrfs kernel code is
>   able to find the next raid members and compose the final array.
>
>> Here is my blkid which will give you an overview of the partitions involved.
>> NOTE that with btrfs there are often multiple partitions with the same LABEL
>> and UUID, but only ONE of those will be the mount point and that mount point
>> can change at any time to one of the OTHER partitions sharing the same
>> label.  If umount OR findmnt happen to get the wrong mountpoint even with
>> the right label, things are not going to work out:
>   So I don't see a bug. All what we need is to more explicitly explain
>   to users that conversion from <device|uuid|label> to <mountpoint> is
>   not reliable, because the same device could be mounted on more places
>   or the mountpoint could be connected to more devices.
>
>> /dev/sda5: LABEL="MAGEIA3BTR-BOOT" UUID="63f13151-dd34-45e1-b40d-7ed7d0d0ec4a" UUID_SUB="978f5c02-5e11-4df4-8f72-4ff6d09e58ff" TYPE="btrfs"
>> /dev/sdb5: LABEL="MAGEIA3BTR-BOOT" UUID="63f13151-dd34-45e1-b40d-7ed7d0d0ec4a" UUID_SUB="0587a676-8bad-4d61-a8af-0e262d997ba7" TYPE="btrfs"
>   Yes, I see.
>
>      Karel
>
>