Bug#484823: multipath-tools: /etc/udev/rules.d/multipath.rules breaks LVM snapshots

[Tim Woodall]

On Sat, 7 Jun 2008, Guido Günther wrote:

> On Fri, Jun 06, 2008 at 10:33:36PM +0100, Tim Woodall wrote:
>> The /etc/udev/rules.d/multipath.rules exposes a race that causes LVM
>> snapshots to fail to be created intermittently.
> You mean:
>
> # take care of devmap partitioning
> ACTION=="add", SUBSYSTEM=="block", KERNEL=="dm-*", \
>        PROGRAM="/sbin/dmsetup -j %M -m %m --noopencount --noheadings -c
> 	-o name info", \
>        RUN+="/sbin/kpartx -a /dev/mapper/%c"
>
> Does changing ACTION="add" into ACTION="change" do any good?
>
Yes. That seems to fix my LVM problem.

I'm using the following simple test script:
feynman:~# cat testlvm
#!/bin/bash

for i in $( seq 1 10 ); do
     /sbin/lvcreate -A n -L500M -s -nroot-backup /dev/vg0/root
     sleep 2
     /sbin/lvremove -A n -f /dev/vg0/root-backup
     sleep 2
done

exit 0

With the ACTION=="add" a typical run looks like this: (note that the
machine has to be idle. If there's lots of disk access then the LVM
snapshot seems to work regardless)

feynman:~# ./testlvm
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed
   LV vg0/root-backup in use: not deactivating
   Couldn't deactivate new snapshot.
   Logical volume "root-backup" successfully removed
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed
   LV vg0/root-backup in use: not deactivating
   Couldn't deactivate new snapshot.
   Logical volume "root-backup" successfully removed
   LV vg0/root-backup in use: not deactivating
   Couldn't deactivate new snapshot.
   Logical volume "root-backup" successfully removed
   LV vg0/root-backup in use: not deactivating
   Couldn't deactivate new snapshot.
   Logical volume "root-backup" successfully removed
   Logical volume "root-backup" created
   Logical volume "root-backup" successfully removed

I've never had fewer than 2 failures (20%) running this script with the
multipath.rules script when the machine is idle.

With the add replaced with change, or the udev rule removed completely I've
never seen a failure.

> Do you have a chance to try the packages from lenny? Kpartx is split out
> into a separate package there and the udev rules are much more robust.
> There are packports of this versions available here:
>
> http://debian.bzed.de/etch-backports/devmapper_1.02.24-4~bpo40+1.dsc
> http://debian.bzed.de/etch-backports/multipath-tools_0.4.8-7~bpo40+1.dsc
>
Not easily on this machine. But I can probably test on another machine
if it would help you.

Note that I've not tested if that add->change breaks anything else. I'm
not even sure how to test this udev rule.

Tim.

-- 
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

    http://www.woodall.me.uk/    http://www.locofungus.btinternet.co.uk/