After the upgrade of last week, I didn’t have any major problems : xen 4 seems pretty stable and does its job well. One problem I encountered the other day was about the dom0 balloning. By default, xen sets dom0_min_mem to 196Mb and balloning set to true. This is all and good untill you try to use too much memory for your VMs, squeezing dom0 to its minimum amount of memory and causing all sort of problems. On the xen wiki, they reccomend as best practice to reserve a minimum of 512Mb to dom0 for its operations. This is done but setting dom0_mem=512M on the grub command line and the adjusting enable-dom0-ballooning to no and dom0-min-mem accordingly to the amount of memory you choose.
On debian, you can set the grub command line once for all just by adding in /etc/default/group the conf variable
GRUB_CMDLINE_XEN="dom0_mem=512M"
Another small problem is related to the reboot sequence. Since I’m using lvm on aoe, the default shutdown sequence (network down first, lvm later) is not going to work for me. As I’ve few lvm volumes on aoe and others on the physical disk, the proper solution to this problem is to write a custom shutdown script for the aoe lvm volumes and make it run before deconfiguring the network interfaces. In the mean time, to avoid the kernel hanging there forever, I’ve added these lines to in /etc/syscntl.d/panic.conf
# Reboot 5 seconds after panic
kernel.panic = 5
# Panic if a hung task was found
kernel.hung_task_panic = 1
# Setup timeout for hung task to 300 seconds
kernel.hung_task_timeout_secs = 120
This will instruct the kernel to panic and then reboot if there a task will not respond for more then 120 seconds.