LVM snapshotting missing symlinks

[Kees Cook]

Binary package hint: udev

The gutsy udev seems to not handle LVM snapshots device symlink creation. Or, if it's a race, I always lose. :)

[Kees Cook (kees)]

First, create a test LV to serve as the "original" LV

# lvcreate -L1G -n original systemvg
  Logical volume "original" created
# ls -la /dev/systemvg/original
lrwxrwxrwx 1 root root 29 2007-05-27 09:59 /dev/systemvg/original -> /dev/mapper/systemvg-original
# ls -la /dev/mapper/systemvg-original
lrwxrwxrwx 1 root root 8 2007-05-27 09:59 /dev/mapper/systemvg-original -> ../dm-21
# mkfs.ext3 /dev/systemvg/original
# mount /dev/systemvg/original /mnt/test
# echo "original" > /mnt/test/what-am-i.txt
# umount /mnt/test

Now, create a snapshot named "duplicate" from the "original" LV

# lvcreate -s -L1G -n duplicate /dev/systemvg/original
  Logical volume "duplicate" created
# ls -la /dev/systemvg/duplicate
lrwxrwxrwx 1 root root 30 2007-05-27 10:01 /dev/systemvg/duplicate -> /dev/mapper/systemvg-duplicate
# ls -la /dev/mapper | grep duplicate
lrwxrwxrwx 1 root root 8 2007-05-27 10:01 systemvg-duplicate-cow -> ../dm-24
# ls -la /dev/mapper | grep original
lrwxrwxrwx 1 root root 8 2007-05-27 10:01 systemvg-original-real -> ../dm-23

At this point, things are very strange: neither the source nor the target
symlinks exist in /dev/mapper.

Checking on the newly create dm-* devices, we find:

# vol_id /dev/dm-21 | grep UUID=
ID_FS_UUID=341cab43-9dc2-4c45-97fd-5b9df09dd748
# vol_id /dev/dm-22 | grep UUID=
ID_FS_UUID=341cab43-9dc2-4c45-97fd-5b9df09dd748
# vol_id /dev/dm-23 | grep UUID=
ID_FS_UUID=341cab43-9dc2-4c45-97fd-5b9df09dd748
# vol_id /dev/dm-24
/dev/dm-24: unknown volume type

So, as I understand it, the mapping is:

dm-21 -> systemvg-original
dm-22 -> ??? (should be "systemvg-duplicate")
dm-23 -> systemvg-original-real (an LVM by-product)
dm-24 -> systemvg-duplicate-cow (an LVM by-product)

The LVM by-product devices are not for general consumption, which leaves
dm-22 as the likely candidate for being the snapshot. (Which is also
supported by the udevmonitor output log -- attached.)

# mount /dev/dm-23 /mnt/test
mount: /dev/dm-23 already mounted or /mnt/test busy
# mount /dev/dm-24 /mnt/test
mount: /dev/dm-24 already mounted or /mnt/test busy

Just to be sure, change the "what-am-i.txt" file:

# mount /dev/dm-22 /mnt/test
# cat /mnt/test/what-am-i.txt
original
# echo "duplicate?" > /mnt/test/what-am-i.txt
# umount /mnt/test

Drop the "duplicate" snapshot:

# lvremove /dev/systemvg/duplicate
Do you really want to remove active logical volume "duplicate"? [y/n]: y
  Logical volume "duplicate" successfully removed

Strangely, the source symlinks have returned:

# ls -la /dev/mapper | grep original
lrwxrwxrwx 1 root root 8 2007-05-27 10:07 systemvg-original -> ../dm-21

We verify that the original LV is unchanged:

# mount /dev/systemvg/original /mnt/test
# cat /mnt/test/what-am-i.txt
original
# umount /mnt/test

So, after "lvcreate -s" finishes, the problems are:

1) the source LV symlink has vanished
   ("systemvg-original" should still exist)
2) the target LV symlink isn't there
   ("systemvg-duplicate" should exist)

[Kees Cook (kees)]

Strangely, disabling /etc/udev/rules.d/65-dmsetup.rules leaves the /dev/mapper/systemvg-duplicate file (though /dev/mapper/systemvg-original still vanishes).

[Kees Cook (kees)]

Ah-ha! dmsetup's rule was ignoring the final snapshot devices (once they were set up correctly), which meant that the final device state lacked a symlink, and udev prompted deleted it. On initial creation, it has a DM_TARGET_TYPES of "linear".

udev_event_run: seq 3143 forked, pid [24597], 'change' 'block', 0 seconds old
run_program: '/sbin/dmsetup export -j254 -m26'
run_program: '/sbin/dmsetup' (stdout) 'DM_NAME=systemvg-testsnap'
run_program: '/sbin/dmsetup' (stdout) 'DM_UUID=LVM-4R9E1MV0xLpEsR74zV3oOmJorMhb2Erw3tmXSDwEMd4nl2BxRjV429fgGeCPXKOB'
run_program: '/sbin/dmsetup' (stdout) 'DM_STATE=ACTIVE'
run_program: '/sbin/dmsetup' (stdout) 'DM_TABLE_STATE=LIVE'
run_program: '/sbin/dmsetup' (stdout) 'DM_OPENCOUNT=0'
run_program: '/sbin/dmsetup' (stdout) 'DM_LAST_EVENT_NR=0'
run_program: '/sbin/dmsetup' (stdout) 'DM_MAJOR=254'
run_program: '/sbin/dmsetup' (stdout) 'DM_MINOR=26'
run_program: '/sbin/dmsetup' (stdout) 'DM_TARGET_COUNT=1'
run_program: '/sbin/dmsetup' (stdout) 'DM_TARGET_TYPES=snapshot'
run_program: '/sbin/dmsetup' returned with status 0
udev_rules_get_name: no node name set, will use kernel name 'dm-26'
udev_device_event: device '/block/dm-26' already in database, cleanup
name_index: removing index: '/dev/.udev/names/dm-26/%2fblock%2fdm-26'
name_index: removing index: '/dev/.udev/names/mapper%2fsystemvg-testsnap/%2fblock%2fdm-26'
name_index: removing index: '/dev/.udev/names/disk%2fby-id%2fdm-name-systemvg-testsnap/%2fblock%2fdm-26'
name_index: removing index: '/dev/.udev/names/disk%2fby-id%2fdm-uuid-LVM-4R9E1MV0xLpEsR74zV3oOmJorMhb2Erw3tmXSDwEMd4nl2BxRjV429fgGeCPXKOB/%2fblock%2fdm-26'
udev_node_add: creating device node '/dev/dm-26', major = '254', minor = '26', mode = '0660', uid = '0', gid = '6'
udev_node_mknod: preserve file '/dev/dm-26', because it has correct dev_t
name_index: creating index: '/dev/.udev/names/dm-26/%2fblock%2fdm-26'
udev_node_update_symlinks: update old symlink 'mapper/systemvg-testsnap' no longer belonging to '/block/dm-26'
udev_db_get_devices_by_name: no index directory '/dev/.udev/names/mapper%2fsystemvg-testsnap': No such file or directory
update_link: found -1 devices with name 'mapper/systemvg-testsnap'
update_link: no reference left, remove 'mapper/systemvg-testsnap'
udev_node_update_symlinks: update old symlink 'disk/by-id/dm-name-systemvg-testsnap' no longer belonging to '/block/dm-26'

This would need to be fixed even after "udev-lvm-mdadm-evms-gutsy" is solved, since the "final" device will have the "snapshot" type, and will need device nodes created.

[Kees Cook (kees)]

 devmapper (2:1.02.18-1ubuntu3) gutsy; urgency=low
 .
   * Adjust debian/dmsetup.udev rule to not ignore "snapshot" devices. This
     will be needed even after "udev-lvm-mdadm-evms-gutsy" is solved (LP:
     #117225).
   * Update Maintainer fields for Ubuntu.

[Kees Cook (kees)]

devmapper (2:1.02.18-1ubuntu3) gutsy; urgency=low

  * Adjust debian/dmsetup.udev rule to not ignore "snapshot" devices. This
    will be needed even after "udev-lvm-mdadm-evms-gutsy" is solved (LP:
    #117225).
  * Update Maintainer fields for Ubuntu.

 -- Kees Cook <email address hidden> Sun, 27 May 2007 12:12:22 -0700