[Xen-users] Xen + LVM + drbd crash

[Thomas Bader <thomasb@xxxxxxxxx>]

Hi all

I setup two Xen servers with Debian etch. Both servers have
800gb of storage. I decided to split this storage into two
partitions (each with 400gb) and mirror the two partitions
through drbd. I created a LVM PV/VG on top of each drbd
block device. I also added Heartbeat to the setup with some
self made scripts.

As a result I now have two Xen servers, each one being
primary for one of the two drbd devices and some DomUs. In
case of a failure the fail-over works fine, resulting on one
server taking over all the DomUs.

During testing I found out that it's easily possible to
crash a server if I run bonnie++ in one of the DomUs. The
DomU has 300MB RAM assigned and I invoke:

  bonnie++ -d . -s 1024 -u nobody

Several seconds after the command my whole Xen server (Dom0
+ DomUs) just hangs. I'm not able to enter any more commands
on the local console, neither am I able to login through
SSH. The machine still answers Heartbeat packets.

The same happens if I run other, rather disk I/O intensive,
benchmarking tools inside the DomU.

It looks like the problem is specific to running the DomU on
a VG that runs on drbd. I tried several other cases with the
same benchmarking tool invocations:

- DomU with an image file stored on the servers /-filesystem
  (which is not inside LVM and drbd): no crash
- running the benchmarking tools on a filesystem created on
  a LV and mounted in the Dom0: no crash
- running the benchmarking tools on a DomU running on LVM
  without drbd: no crash

Somehow, only the combination drbd + LVM + DomU running in
on a VG there seems to trigger this problem. Has anyone of
you experienced the same problem?

Currently, I'm a bit confused about how to further debug
this problem. Does anyone of you has advice about how to
further debug this problem (ie, adding some debug switches
to Xen to see why the whole machine crashes)?

Regards, Thomas.

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